How would you feel if your ex dated your friend?

Would you ever date one of your friends ex’s?
That’s going on right now in my life and I still have some feelings for this guy too. My thoughts on it are kinda like “Fine if she wants my sloppy seconds then go for it” but I can’t figure out why it hurts so damn bad.

They also tried to hide the fact that they were dating.

I’m not sure flutter, how would you all react and do you think its taboo?