Really?

NO I'M NOT OKAY WHEN IT COMES TO JOKING AROUND ABOUT OUR PAST RELATIONSHIP. YOU FUCKED MY BEST FRIEND AFTERWARD.

So, no, it's not alright and quite frankly will NEVER be alright to joke around about.

Stop giving me hard time about it. I'm not being unreasonable. I don't want to discuss anything about the past.

I'm not sad about it. I don't still have feelings. I'm not bitter. It's just bullshit, and I'm not cool with discussing it even casually.

Just leave it at that.
I have enough going on right now and I don't need anything else getting me riled up.