Im not a father, so I cant say this with fairness, but weve all be young once.
Girls are PROBABLY going to have sex with boys. Its not the boys fault, the girl wants sex. If you don't want your daughter to have sex with a boy, simply be realisitic/fair with her. Shes not going to be a virgin all of her life anyways, she wont be a kid forever, realisitcally its going to happen.
As for catching her in the act, if you havent talked/told her not to, its your fault, not hers. If you did tell her NOT to do it, shes disobeying you and you should probably do something to discipline her, but not make her feel like you put a chastity belt on her. I know Ive dealt with a lot of "NO SEX EVER" parents, mine/person Im dating, and it doesnt really help. All it does is make kids have less-safe sex in less-safe places.