How would you react if your best friends started sleeping together without telling you? What about if one was your ex, and the other was your other best friend's ex?

Wouldn't bother me... they wanna knock boots, it's not my business, more power to em.

Though I'd be skeeved at having to have any kind of discussion or contact with my ex, since he turned all stalker-weird on me when I broke it off.... I'd say if it was one certain of my BF's exen and mine.. they fucking well deserve one another.

Have at it.