Why on Earth? Virgin sacrifice
Why are witches and Satanists so obsessed with sacrificing virgins?
Because they are of inestimable magical value, of course. And they’re valuable because, at heart, the most licentious Satanist is as puritanical as any Evangelical Christian, and in full agreement with your gran that sex is intrinsically sordid and staining.
The whole point of a sacrifice is to offer something of value to your god. The pure and untouched are supposed to make better gifts, either because they are themselves somewhat godlike or (depending on your choice of deity) because it would be an insult to present a woman who has already been ravished to a god who is looking forward to ravishing her himself.
It’s worth noting, though, that practitioners of some forms of magick (that’s the sexual variety of ordinary old magic, for the uninitiated) actually prefer their women sexually experienced. Aleister Crowley, the infamous Great Beast, kept a string of near-prostitutes to help out with his sexually-fuelled rituals. They were known collectively as ‘Scarlet Women’, though one of his favourites, Leah Hirsig, was also dubbed - rather unkindly, I feel - ‘The Ape of Thoth’.