I've been thinking about this for a few weeks now - years ago, I read a tankoubon, and one of the first chapters featured this boyfriend and his really shy, average looking girlfriend. To impress him, she changes her looks before a date, but winds up getting hit on by another guy, before her boyfriend comes in and saves her...or maybe she runs away and he finds her? I don't remember. And then they end up having sex on the roof or something.

This ring a bell for anyone?

Well, this certainly reminded me of something, but I had to leave it mulling for a while since it's been a while since I've read it. Something like this happens in an early chapter of angraecum, namely the second chapter, titled Gloxinia. It isn't an exact match to your description, but it does fit the general idea.

[img]http://i2.kym-cdn.com/photos/images/original/000/028/438/5700.necropost4kobf7.png[/img]