Stan is being haunted by malevolent demonic forces because his vengeful ex of your choice placed a curse on him (maybe he experiences pain whenever he orgasms? silly stuff like that) He goes to Tweek(some sort of witch/spirit medium??) for help and the only way out of the curse is a freaky ritual which ofc involves sex and Stan having to prove himself as an adequate sexual/romantic partner lmao.

Tweek is conflicted bc he does want to comfort Stan and be gentle but Stan is testing his patience with his bs probably

Top/Bottom doesn't matter! Sorry this is kind of vague sexually