Man, don't have sex just because it's expected of you. If both of you are into it, she'll be wet enough and you'll be able to do her just fine. Plus, you'll have an idea where things fit once you *really* get into it with her.
Yeah, sometimes it's uncomfortable the next day (so I've been told) but my understanding is that it isn't like getting your nads kicked so don't worry. The girl is just as uncomfortable the next day too. But all that does is make you think of more sex so how bad can *that* be?
I *still* get a little sore the next day but it's been a while between guys. If it was a regular thing, it might be a different story.
Oh. And wear a rubber like the others said.
Good luck.
Tara