Particularly incompetent and/or rookie villains will usually kidnap a girl known to be a hero's girlfriend just to spite him-ignoring the obvious risk inherent with such a victim-and ignore claims from a pleading victim that she doesn't qualify, only to eventually learn their mistake the hard way. Of course, the victim's actual purity tends to be directly proportionate to the intelligence (or lack thereof) of the ones making the offer, and inversely proportionate to the consequences for offering a non-virgin.
(If said member of the party is, in fact, not a virgin, you can expect some hilarious hijinks to occur when that fact is uncovered.) If not, then they're usually a Love Interest or a member of the heroes' party who got kidnapped and carted away for sacrificial purposes. Virgin Sacrifices are usually young (but almost always past puberty), female, conventionally-attractive, pure-minded, and sometimes raised from birth to accept-even celebrate-their honored position as Virgin Sacrifice.