It's either a bug or the corruption meter acts as a "pent up" meter. So when she has sex, it goes down and she won't do things she'd normally do if it was high.
maybe its post nut clarity, like oh no im a whore and just fucked a creep my relationship status cant get better than wary with, maybe i should reconsider my life choices, corruption down.
whereas the male mc was, ooh got my dick sucked by some strange, not very satisfied we ended it right there though the alley was kinda hot, corruption up.
simply an assumption, but thats what it felt like as i played the scenes and read the inner monologues. also from experience, ive gottan bjs that only made me hornier and more willing to do something stupid and also had sex with a person i wasnt really interested in and suddenly felt like becoming a nun afterward even though im a dude. so the corruption changes kinda make sense to me.
on a different note, best thing about nuns, if you dont look em in the face or undress them, fucking one is like fucking em all. which is also the worse thing when you look a really ugly old one in the face months later and she winks at you. im not saying any of this happened, but if you can picture it than you know what i mean. but seriously, theres only like 2 vns with decent nun scenes, and 3rd where itll be possible in future updates. sad stuff. i wont name em just on a random nun tanget atm.