Gender preference
Gender preference was introduced in The Sims 2 and carried forward into The Sims 3. Each Sim who is a teen or older has a set gender preference that is defined by the averaged total of all their romantic interactions with other Sims. For instance, if the majority of a Sim's romantic interactions have been with females, that Sim will have a preference for females, and vice versa, if the majority of the Sim's romantic interactions have been with males. Gender preference can be changed over time; a female Sim with a preference towards females who has enough romantic interactions with male Sims over time may develop a preference for males. It is possible to influence the gender preference of townies and uncontrolled Sims by performing minor flirt interactions such as checking them out or whistling at them.
Newly created Sims, whether freshly out of CAS, or newly created townies, have a neutral gender preference, though some pre-made Sims have pre-set preference levels. At least in The Sims 2, Sims who are born in-game or are adopted start their teen stage with a neutral gender preference.
Gender preference affects some gameplay mechanics. In Nightlife and later, Sims only show chemistry for Sims of their preferred gender, and only Sims of the preferred gender will be considered when a Sim uses the "Scope Room" self-interaction. In addition, the interaction may not be available to Sims whose preference has not been nudged off of neutral. In Generations, when a Sim throws a bachelor/bachelorette party, the gender of the entertainers will based on the gender preference of the host.
A player-controlled Sim can be commanded to perform romantic interactions regardless of gender preference, though it appears to be more difficult for romantic interactions in The Sims 3 to succeed if the Sim being pursued has a gender preference not associated with the pursuing Sim.
Gender preference is a hidden aspect of Simology that can normally not be seen without the use of cheats, mods, or programs such as SimPE.