Had a female friend once who had just divorced. She told me their sexual relationship was pretty much dead and she had lost her interest. So her husband wanted to divorce her and found a new girlfriend. But here's the interesting part -- apparently because of the tension of the upcoming divorce and "competition" from his new girlfriend, my friend said she was hot to have sex with her soon-to-be ex. And they did, and it was some of the hottest sex she'd ever had.
I understand the psychology. Kind of sad though. Not my preferred way to spice up my love life.