Same-sex relationship

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In The Sims series, every Sim is technically neutral, 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).

There aren't any premade homosexual couples or families in the series, although some single Sims are lightly implied to be homosexual in their bios. However, all Sims aged teen and above, both premade and player-created, can engage in both heterosexual and homosexual romantic relationships. Players can also create married/joined same-sex couples in Create a Sim.

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 heterosexual and homosexual 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.

The Sims 2
In The Sims 2, there is the option of formalizing any romantic relationship, but with different titles for same-sex couples (Joined Union) and opposite-sex 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.

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.

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 formalised homosexual relationships in the form of Joined Union, identical to heterosexual 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 homosexual 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 homosexual couples to have children that are their biological offspring, although they have always been able to adopt in the same way as heterosexual 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. However, in The Sims 2 and The Sims 3, cheats and 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. The only exception occurs in The Sims 2, as an adult male who is abducted by aliens will be pregnant with an alien baby when he returns, and the other biological parent will be the male NPC Pollination Technician. There are, however, mods that allow this.

Pre-made (possibly) homosexual 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 homosexual, 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 on his family tree, suggesting Gaylord may have adopted him. 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" as slang for "homosexual".
 * Using boolprop testingcheatsenabled or SimPE in The Sims 2 allows players to view their 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 a heterosexual romantic storyline. The most commonly cited example is that of Kent Capp, who has a slight preference for males at the beginning of the game, despite an implied budding romance with Bianca Monty. A lesser known example is Jason Cleveland, who also has a slight preference for males, despite being married to Marissa Cleveland at the start of the game. An almost not known example is Alon Livingston, who is a newspaperboy in the game.
 * Similar to the instances above, a few premade Sims in The Sims 2 are bisexual, having equal preferences for both males and females. Two examples of this are Circe Beaker and Titania Summerdream, while Ariel Capp is predeterminated to be bisexual when she ages up to teen. Interestingly, the only preset bisexual Sims in The Sims 2 are females.
 * At least one player has loaded the premade female character Constance Shelley in Riverview and found her to have a romantic attachment to another premade female Sim. This may be deliberate on the part of the game's creators, or may be a result of the Story Progression feature in The Sims 3, which allows Sims to (amongst other things) form romantic relationships without the input of the player. [This in itself may suggest that, unlike in the original game, Sims in The Sims 3 will default to bisexual rather than heterosexual behaviour if they are allowed to act autonomously.]
 * In The Sims 3, if Story Progression is on, Ayesha Ansari and Dorothy DeMayo (a homeless Sim) are often found in same-sex-relationships. Ayesha Ansari will often have an interest in one of her roomies, and she rarely has a heterosexual relationship if the player isn't playing with her.
 * 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.
 * Some players have interpreted ambiguous statements in the family bios of the Friends household (in The Sims 2: University) and the Roommates household (in The Sims 3: Ambitions) to mean that the individuals in those households may begin a homosexual relationship together. In both cases, the households consist of two same-sex roommates living alone together, perhaps adding some credence to the theory; however, it is still purely speculative.
 * This site mentions several characters who often seem to be considered to be homosexual by players, including the abovementioned Pascal Curious and Nervous Subject, as well as Beau Broke and Frances J. Worthington III.