I had issues with the romance myself, but then had to ask myself if I was thinking it was unbelievable simply because she was thin and beautiful, and he was dumpy and clutzy. I had to ask myself if it was possible for a 'pretty' girl to fall for an oh-so-'regular' guy, and how much of the cultural stereotype I was allowing myself to be manipulated by.
Then of course, you have to deal with the backswing to that, and whether or not you're overthinking things, or trying too hard to be non-stereotyping.

All of which really kind of took me out of the movie.

In the end I accepted the premise, mostly in the interest of attempting to enjoy the movie. (Because I do love a movie with a happy ending. That escapism is all about why I watch most movies. I can swallow some relatively outrageous premises in order to allow the movie to succeed.)

It would have been a better (and easier to accept) movie if they'd done a better job of character development before the one-night stand.