Kind of a joke, but...

[Thajocoth]

Well what keeps the Elan from overpopulating the world?

The Belkar, obviously...

If a player in a game of mine decided that their character has sex, I'd roll a bunch of d20s, ask their Endurance score and their Fort Defense, and probably not say another word... Just 'cause it'd be funny. Later, if I decided I wanted the plot hook, I'd claim the dice did it, which would ultimately be a lie. I would roll for lesser details of the situation though... Roll a (very low for most NPCs, but high if they picked an obvious skank) chance that the NPC had something to catch, and if so an attack against Fort to see if they catch it. (I like using the disease system.) If I take the pregnancy plot hook, roll a d20: 20 = twins. Stuff like that.