Well, 1. a virgin has moral value. She hasnt given herself to anyone, that means it has to be someone quite unique for her to give her virginity. They wanna be that person. Another: no STDs. Plus, she wont think any other guy has been better because she hant been with any other guy.
OnTheFlipSide is pretty much right. Guys want to show the girl what sex really is and make their first time their best time. But, some guys would rather not have sex with a virgin because they do not want to have to teach her(if you will) how to have sex. Check MTV2's "Guy Code", episode 29 season 3, which aired 01-22-13.