I personally stand by the theory it didn't evolve for sexual reasons but rather for the infant's safety. It's much easier to clean a male's genitalia than it is a female's. The hymen is thus in place to keep foreign matter out (fecal, dirt, diseases, whathaveyou). As a female gets older they learn how to clean themselves thus the hymen becomes less necessary to have intact.
I can't see it being a reliable gauge of virginity considering a recent study showed 52% of a group of adolescent girls having an intact, non-disrupted hymen after first sexual intercourse. Though some have claimed that it can also let a man know that he was the first to have sexual intercourse with the woman, the evidence suggests that isn't always the case. However, the fact that it wasn't broken during intercourse may lend more towards the theory that it is there for protection against foreign bodies. A more elastic hymen is thus more desired than one that breaks just by strenuous exercise (as can happen and has been documented).
I could of course be wrong. And nobody really knows for certain. If they say they do they're probably lying through their teeth. It is ranked up there with the mystery of the Vermiform appendix (which also has multiple theories as to why we have it).
Reference: Adams, J.A., Botash, A.S., & Kellogg, N. (2004). Differences in hymenal morphology between adolescent girls with and without a history of consensual sexual intercourse. Arch Pediatr Adolesc Med, 158(3): 280-285.
bloop