I thought it was sort of contrived and a bit out of character for Cordy, but I could understand the whole it's-the-end-of-the-world thing. I mean, being trapped in a room together while catastrophe ensues is pretty much the #1 reason to have sex on a TV drama.
If I thought it was really and truly apocalypse time, I'd wanna go out on a happy -- especially if it would also give me the chance to give this boy, about whom I cared deeply and whom I knew to have been raised in HELL, his one moment of happiness before he died. Sex is kind of a natural reaction to death. It makes you feel alive. So this one time and in these specific circumstances, I don't think it's very wrong. Instead, I think it's very human.
Of course... if the world doesn't end, it becomes very confusing for everyone, especially for the young, impressionable kid who now probably thinks you love him. And what about pregnancy or STDs? I mean, did they use a condom, or were they like, "Whoops! End of the world!?" lol. Then, there's jealous Daddy Dearest to consider. I mean... it really is wrong on all kinds of levels. That's why they have child molestation laws.
So I'm torn. On the one hand, I probably would have thrown the kid a bone, myself, in those circumstances. Why not? On the other hand, if they survive the night, the list of answers to "why not" becomes long and disgusting and sets everyone up for an Oedipal complex of epic proportions.