Personally, I can't feel much under 5". If I were in a similar situation, since sex is a really important part of a serious relationship to me, I would definitely want to keep this person as a friend, but not as a romantic partner. Of course, as always, different people have different preferences. A few of my girlfriends like smaller guys because they're pretty small themselves. It really depends on the girl... if the guys KNOWS he's "The One" for her and she's confident of the same, some measurement or another shouldn't matter.