You can stop sex at any moment by simply canceling the interaction. Additional functions are available depending on the features you use. Sims in sex can perform a set of actions which involve undressing, applying of cum, usage of a strapon, inviting other Sims to join, changing the sex position, changing the sex location, or swapping the sexual roles.
You can download more sex animations on the Download page. The main way to initiate sex is by clicking on any object that has sex animations available.