It seems like a pretty gaping wide plot hole to me. Especially when she's about to be sacrificed and they explain to her that they need a virgin to perform the ritual. That would have been a good time for her to mention that she lost her virginity in 7th grade.

It also doesn't track that Needy would tell Low Shoulder that she's a virgin after she overhears them saying they're looking for one.

I understand that they needed a plot device to explain why she turned into a succubus, but they could have done that a number of different ways. I think this would have worked better as a vampire movie which it mostly was anyway.