Sex in tabletop roleplaying games can be handled in a number of ways, depending on your group. The vast majority of groups, of course, simply "fade to black", and take it as given that sex occurred and we can all move on to the next scene. In cases where one's performance is somehow important, it's generally accepted in most game systems that whatever is the equivalent of "Charisma" (in this case, "Social") is the relevant stat (unless you have a particularly unusual skill or ability, such as mindreading, or superhuman strength, or the ability to shapeshift).

Of course, there are the sorts of games in which players may choose to go into detail (horrible, horrible detail) about their encounters, either there at the table or in a one-on-one session with the individuals involved. Indeed, there are some games in which this is entirely the point, and the combat and adventure are superfluous.

And there are plenty of games in which the results of this coitus are important outside of the act itself - I've been in games in which two PCs had a romantic relationship and then broke up but remained in the same adventuring party, I've been in games in which enemies attacked while a player was in flagrante delicto, I've been in games where a character got pregnant, et cetera, et cetera.

It all comes down to what sort of group you're in and what sort of game you want to play. Tabletop roleplaying games can be as diverse as - for example - movies. Some games are the equivalent of The Matrix, some are the equivalent of Crank, some are Waterworld, some are Aladdin, some are Milo and Otis, some are Backdoor Sluts 9. As with any artform, you have to know your audience (and, just as importantly, your audience needs to know what they're getting into and be ready to play along).