What about when Harry's about to graduate/just graduated and tells Snape that he's completely in love with him (and has been for forever!!!) and Snape gets really mad and accusses Harry of pranking him? And then they have sex!
Oh! Or when Snape's all, "I'm mean and old and ugly and smelly" and Harry's all, "You're gorgeous and prefect and classically handsome... and yes okay a bit mean." And then they have sex!
Also! What about when Harry's been dosed with some sort of aphrodisiac either through Malfoy/other evil student slipping something to him or dodgy potions accident? And then they have sex!