Gender
In all The Sims games, there are male and female Sims, who can generally be distinguished by their appearance. In general, females have breasts, larger hips, smaller shoulders and waist, and have lower muscle tone. Males lack breasts, have larger shoulders and smaller hips, and have higher muscle tone. Females also tend to have a larger clothing choice at CAS.
Differences and similarities
On the one hand, there are some minor differences between the genders in the games. The most prominent difference is that one male Sim and one female Sim are required to create a baby through in-game interactions, as of The Sims 2. Also, gender-specific pronouns will often be used when describing the Sim in a pop-up or on a tag, such as those that appear for some careers, moods, and traits.
Overall, however, there are few differences, when any, between the genders. For example, reactions of female and male Sims to the same situations appears to be equal, if they have the same personality. Also, both genders may pursue the same jobs.
Children and prior life stages have none of these physical differences, although most of their customization features are separate between genders.
The Sims
Only opposite-sex couples have the "Propose... Marriage" option. Same-sex couples only have the "Move In" option.
In The Sims: House Party and The Sims: Hot Date, there are doors that are used exclusively by one sex, usually used for washrooms on community lots.
The Sims 2
There are some customization features in Create a Sim that are only available to one gender. For example, only male Sims have options for facial hair.
Gender does not typically affect social interactions. For example, during a romantic interaction, the Sim who started the interaction normally assumes the pursuing role, regardless of their gender.
With the new pregnancy feature in The Sims 2, females can get pregnant through WooHoo, and males can get pregnant through alien abduction.
A male Sim stands up to use the toilet, if his Bladder score is high enough.
When a female Sim takes a shower, the game censors her breasts.
There are doors that are used exclusively by one sex, usually used for washrooms on community lots.
Only opposite-sex couples have the "Get Married" option. Same-sex couples only have the "Joined Union" option.
The Sims 3
A lot of gender-related aspects in this version are similar to The Sims 2. One exception is that marriage in The Sims 3 is the same between opposite- and same-sex couples.
In The Sims 3, certain relationship labels are organized by gender.
In The Sims 3: World Adventures, there are doors that are used exclusively by one sex, usually used for washrooms on community lots.
The Sims Medieval
In The Sims Medieval, gender differences are similar to those present in The Sims 3. However, there is a new element that is not present in any of the other games in the series: when babies are born, they are friends with their mother, but they are only acquaintances with their father, unless the father is made to socialize with his offspring or if he is controllable.
Unlike the real Middle Ages, men and women are free to pursue any profession
Gender differences in toys
There are clear gender differences in toys; some are stereotypically thought of as being for girls, and others as being for boys. Regardless, the player can choose whether to have Sims follow these stereotypes.
When Sim toddlers and children make free will choices about which toys to play with, they do not necessarily adhere to these stereotypes: girls do not necessarily pick up "girlish" toys, nor do boys necessarily pick up "boyish" toys.
Gender differences in NPCs
In The Sims 2, there are more female maids and social workers than males, and more repairmen than repairwomen.
In The Sims 3, there appears to be gender equality in NPC jobs: for each type of NPC, there is one male and one female.