Personally I don't necessarily make my characters virgins, unless they're very young (and those stories no longer get posted here). But I think the biggest reason to use (
) a virgin as the victim is you can portray a greater sense of fear on the unknown.
A woman who is experienced might have some fear over the unknown aspect of her attacker, what will he do during the rape, what will he do after the rape... but a virgin has those fears, plus the worry of what sex is and how her greatest treasure... her virginity... is being stolen from her.
Just my point of view.