Why do (some) people who have affairs with someone who's married seem to wear it like a badge of honor?

I get that there are cases of people in sexless marriages who stay together for practical reasons and don't really care about each other anymore, so both partners go off and do their own thing. That's sad, but at least no one's really getting hurt. What really bothers me are the situations where deception is involved. The ones who are proud of that disgust me. I find them loathesome, subhuman filth.