Because I have! Love DH a lot but sex was sporadic and duty rather than desire.
I hooked up with this guy online, we messaged, he declared his feelings, I resisted for a few months then gave in. This has gone on for a year.... and as a result everything at home has got better. Better sex, more caring, more fun.
We both agreed that we are happy with our spouses but love the frisson of the affair. There are no plans on either side to leave and the thing is complementary rather than replacing.