Culkin seemed extremely awkward (well maybe I just related to him more as a guy lol), but she was not at all acting like she didn't know what to expect.
Amy: I swear to God...I swear to God! That is NOT how you treat your human!
Well, as a guy, remember puberty? How we were when we were virgins? We knew everything about sex. Well, basically everything except doing it.
Girls aren't different. Just because they're virgins doesn't mean they don't know anything about it at all and would faint at the sight of a penis or something.
She was just the more confident one of the two. Being the attractive one (and she knows it), with Rory basically just the nerdy kid who had a crush on her since forever.