I think part of the response someone feels correlates directly to how long, and/or how serious, the relationship they are leaving was.
I've been married for almost 13 years (wow, didn't think I was that old
), and am pretty happily married at that. If something drastic happened soon, I don't think having sex with someone else would be the first thing on my mind. But I would also say that, even after all those years of monogamy, sex still isn't just a physical act to me. So I'm sure that impacts the way I think I'd feel about it.
But I can easily see how someone who was either already distanced emotionally, hadn't been fully invested, was already unhappy in the relationship, or who was angry at the other person, would have that mindset.