The bittersweet, passionnate, lusty sex you have with your partner shortly after, whilst, or shortly before breaking up with them. Usually both parties are aware that they're breaking up.
It's too bad she left me, but man, was that some good breakup sex.
When the relationship with your partner has just ended, but you have sex one last time before you say goodbye.
Doctor: Sir, I'm sorry your wife but your wife didn't make it through surgery
Joe: Oh my God! What am I going to do?
Doctor: Would you like to see her and have Breakup Sex before we move her to the morgue?
Joe: I suppose for ole times sake why not
When your girl wants to pull some shit and say you're broke up. Then you're at work and get a text like
"Well I mean damn can't you picture it - me in my bustiers on my knees with you in my 👄 and then me licking and sucking getting you 🤑 and then pulling me up & bending me over 🍆you sliding my pink cute thong over and taking me bent over the bed .... Hot I mean hot .... Oh well that's a shame😜 "
I thought that chick was gone for good, next thing you know we're breakup sexting.