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