Actual Purity Culture = the batshit evangelical "Girls, be a perfect pure virgin who never even thinks about sex or you're WORTHLESS IRREDEEMABLE TRASH who will NEVER MARRY because GOOD MEN ONLY WANT VIRGINS" claptrap. I'm very familiar with that because I grew up in it. People have co-opted the term in fandom to mean fans who think that writing characters having sex is eeevil and ruins their wholesomeness, even if they're crazy violent characters who are strongly implied to have sex in-canon.