Same-sex relationship

A same-sex relationship is a sick and demented form of relationship in The Sims series. Players have been able to create same-sex relationships since the original game, but the way the game has treated these relationships has changed over time. Currently, there is one pre-made same-sex couple in The Sims series. However, several other Sims are implied to be gay or lesbian in their biographies, and some Sims are rumored to have these tendencies if they are allowed to act with free will.

Attraction
In The Sims series, every Sim is technically bisexual, as he/she can be directed to have a crush on, fall in love with, engage in romantic interactions and WooHoo with a Sim of either gender, provided they are of the appropriate age group(s).

In The Sims 2, while editing a Sim in SimPE, a preference scale for males and females, can be found. This was carried forward into The Sims 3, but is not as accessible. When first played, most Sims either have a completely neutral preference or a slight preference for (or rarely against) one gender. A Sim's preference scores gradually change as he/she interacts romantically with other Sims during gameplay. Therefore, a Sim's gender preference is fluid rather than fixed.

The Sims 2: Nightlife introduced a system for chemistry between two Sims, which is present in all later expansion and stuff packs for The Sims 2. While gender plays no intrinsic role in the chemistry between two Sims, it is possible to mildly influence a Sim's gender preference by choosing facial hair (only available to males) or make-up (available to both sexes, but very rarely worn by male pre-mades) as a turn-on or turn-off.

In The Sims 3, it is rumored that having a same-sex couple in the active household will increase the chances of other Sims in the neighborhood's inactive households autonomously forming same-sex relationships. However, occasional same-sex relationships do develop autonomously regardless, though not so prevalently.

The Sims
In The Sims, same-sex couples can move in together, but cannot marry. However, marriage is not a recognized state in the game, only a brief social interaction, and no formal relationships exist between Sims. Therefore, once the "Marriage" or "Move In" interaction is completed, the game no longer distinguishes between gay or straight couples.

All households containing two or more adult Sims will occasionally receive a random phone call asking them if they want to adopt a baby. Due to the lack of formal relationships between Sims, any adults living in the household can then be regarded as the adopted baby's parents, at the discretion of the player.

In the console spin-off The Urbz: Sims in the City, there is a mission that actively requires the player character to become involved in a love triangle with a male and female Urb, though the player may choose one over the other.

The Sims 2
In The Sims 2, there is the option of formalizing any romantic relationship, but with different titles for gay and lesbian couples (Joined Union) and straight couples (Marriage). Same-sex couples can go steady with one another and get engaged in the same way as opposite-sex couples.

All adult and elder Sims who have not lost a child to the Social Worker can adopt by using the Call... Services option on a telephone. If the Sim who calls the adoption agency is married or joined, their partner will be considered the co-parent of the adopted child, and will receive a line on the family tree. Members of a joined same-sex couple may roll a want to adopt.

When interacting with the Gypsy Matchmaker, Sims will be given the option to choose a male or female date, regardless of their gender preference. Similarly, wishing for love from the Genie will result in a dialog box giving the Sim the option to choose the gender of their prospective lover.

The Sims 3 and The Sims Medieval
In The Sims 3 and The Sims Medieval, marriage is available to any couple in a romantic relationship, and Sims of either gender will recognize their spouse as "husband" or "wife". As in The Sims 2, same-sex couples can go steady with one another and get engaged in the same way as opposite-sex couples. If Generations is installed, two teens of the same sex may autonomously go to the prom together as dates.

In The Sims 3, all young adult, adult, and elder Sims who have not lost a child to the Social Worker can adopt by using the "Call for Services" option on a telephone. If the Sim who calls the adoption agency is married, their partner will be considered the co-parent of the adopted child, and will receive a line on the family tree. Adoption is not available in The Sims Medieval.

The Sims 4
In The Sims 4, same-sex relationships are possible, as in the previous games.

Problems faced by Sims in same-sex relationships

 * In the original The Sims, Sims with a romantic attachment to a same-sex Sim not already a member of their household could not ask that Sim to marry them. They could be invited to move in as a roommate, but there was no differentiation between a same-sex love interest and a platonic friend of either gender. However, since marriage in The Sims was largely informal, the effect on game-play was minimal.
 * The Sims 2 allowed formalized gay and lesbian relationships in the form of Joined Union, identical to straight Sim marriage in everything but terminology, although some players have observed that a Joined Union may yield fewer aspiration points than a Marriage for a Sim with a related want. However, numerous player-created hacks are available that allow same-sex couples to marry in The Sims 2 on the same terms as opposite-sex couples, and in The Sims 3 all couples are able to marry, regardless of gender. However, if the player creates a same-sex couple in CAS for The Sims 3, the relationship the couple has will be lower than heterosexual couples. It should also be noted that while family members of a Sim in a same-sex relationship will get a memory of the family member getting engaged (provided that he or she did in fact get engaged) they will not receive a memory of said family member getting married, or 'joining union' with a member of the same sex.
 * None of the games in The Sims series allow gay or lesbian couples to have children that are their biological offspring, although they have always been able to adopt in the same way as straight couples or single Sims. In The Sims Medieval, same-sex couples cannot have any children together, as the option to adopt does not exist in this game. In The Sims 2 and The Sims 3, cheats and/or hacks can be used to allow a same-sex couple to have a baby that is the biological offspring of one or both of them. Additionally, in The Sims 2, an adult male who is abducted by aliens will be pregnant with a half-alien baby when he returns; the other biological parent will be the male NPC Pollination Technician. (While alien abduction and impregnation is possible in The Sims 3: Seasons, the abducted male will not be a biological parent, simply a host.)
 * Some spin-off games with story modes which allow the player to choose a spouse for their character (such as The Sims Castaway Stories) are sometimes reported as only allowing opposite-sex relationships in order to complete the game. However, some other games (such as The Sims Bustin' Out for console) do allow the player to choose a same-sex partner and still complete the story objectives.

Pre-made (possibly) gay, lesbian or bisexual Sims

 * Gobias Koffi, one of the premade Sims living in Sunset Valley (the only neighbourhood that shipped with The Sims 3) is implied to be gay, as is his late father, Gaylord. Gobias's bio hints that he has been unable to meet the right woman because he may in fact prefer the company of his male friends. Gobias has no mother in his family tree, suggesting Gaylord may have adopted him. It's possible his mother may have died, but this would have likely been hinted at. Both of their names also offer hints as to their possible sexual orientation: Gobias appears to be named in honour of a sexually ambiguous character from TV's Arrested Development, while Gaylord's name presents an obvious, though not necessarily intentional, pun on the word 'gay.'
 * Using testingcheatsenabled, the InSimenator, or SimPE in The Sims 2 allows players to view Sims' degree of attraction to both genders, which determines how likely they are to accept or reject a romantic advance from a Sim of that gender. Players have commented that several Sims show a preference for their own gender, often despite being involved in an opposite-sex romantic storyline - this is particularly prevalent in Veronaville, where most of the pre-made males have at least some romantic interest in their own gender. The most commonly cited example is that of Kent Capp, who has a preference for males at the beginning of the game, despite an implied budding romance with Bianca Monty and being the subject of Titania Summerdream's crush. A lesser known example is Jason Cleveland, who also has a preference for males, despite being married to Marissa Cleveland at the start of the game.
 * Similar to the instances above, a few premade Sims in The Sims 2 are bisexual, having equal positive preferences for both males and females. Two examples of this are Circe Beaker and Titania Summerdream, while Ariel Capp is predetermined to be bisexual when she ages up to teen. Interestingly, the only preset bisexual Sims in The Sims 2 are females.
 * In The Sims 2, according to Nervous Subject's gender preference, he is neutral toward males and repulsed by females. Since he has good relationships with the Curious brothers, and always shows up on the Curious family lot while Pascal Curious is giving birth, many players have paired him up with Pascal. This may have been intentional on the part of the developers, though that is speculative. However, in The Sims 2 for PSP, he was in an opposite-sex relationship with Annie Howell.
 * TV Tropes mentions several characters who often seem to be considered to be gay by players, including the abovementioned Pascal Curious and Nervous Subject, Beau Broke and Frances J. Worthington III, and Ripp Grunt, who seems often to be regarded as bisexual and romantically interested in both Johnny and Ophelia.
 * In The Sims 2: University, the Friends household consists of Mitch Indie and Max Flexor, whose bio contains ambiguously suggestive statements about how they might "discover" new things about their friendship now that they live together. A similar thing happens with the family bio for Blaise Kindle and Alma Drill in The Sims 3: Ambitions, and in addition players have noted that their shared bedroom consists of two single beds pushed together to look like a double bed at first glance, perhaps implying that their relationship is intentionally set up as a possible romance. Both pairs of friends may randomly roll wants for romantic interactions together, a further indication that this may have been deliberate.
 * Viktor Valquist and Jesper Sandstrom, from Aurora Skies, despite being only best friends forever in-game, are strongly implied in the family biography and even on the family picture, to be a gay couple. The surname of Viktor's adopted daughter, Linn Valstrom, is a portmanteau of both men's last names, implying that they are both considered her parents, even though only Viktor is recognised as such in game.
 * Dylan and Audrey Shear of Roaring Heights are the first ever couple in the series who are openly in a same-sex relationship; they are married and have a child together. Additionally, there are two "secret" same-sex affairs taking place in the town: Mark Davis-Welles and Michael Dandy are in love, despite Mark being married to his wife Rita Davis-Welles, who is herself secretly involved with a woman, Virginia Supine.
 * The Prima Official Game Guide for The Sims 2 base game features a comic strip-style tutorial chapter revolving around the lives of three CAS Sims living in Pleasantview. One storyline involves a married female Romance Sim engaging in an affair with Nina Caliente, who is also shown to be involved with Don Lothario. Based on this, some players choose to regard Nina as bisexual, despite the fact that the game guides are not considered official series canon.
 * At one point in The Urbz: Sims in the City  for console, the player character becomes involved in a love triangle with a female Urb, Babbette Couture, and a male Urb, Cash Monet. This implies that the player character is bisexual and, depending on the gender of the PC, that either Babbette or Cash is either bi- or homosexual.