It's that time of the month... You know, when the full moon comes out
All joking aside, that's pretty much the reason. Menstrual blood is a powerful ingredient in magical spells, and, as I've mentioned in one of my threads, the New and Full Moons are both powerful magical thresholds. These thresholds do tend to sync with a woman's menstrual cycle, and virginity in many schools of faith implies purity.
The corruption of a pure thing is like splitting the atom. It denotes a MASSIVE release of energy and a change in states of being.
What better thing to revive the incarnation of Ultimate Corruption?
However, this also implies that Shaft (or his minions) probably did a
particular truly awful and despicable act to the chick being sacrificed that would never be able to be shown on screen in a T-rated game.
After all, to corrupt a pure thing, you have to do
something to corrupt it, and there's really only one way to do that to a sexual virgin.
Unfortunate implications and fridge horror to follow.