My female friend dated a married guy for several months, not sure how it ended though. My ex wife had been dating a married guy, but she took a job on the other side of the country so I havent' heard how that one's turned out. Sex with a married person is often out of loneliness and the need for human contact. Rarely are there couples who have that open relationship which can survive sexual tryst without one sucumming to jealously.
If you're a spiriitual person, you know it's just plain wrong along most religious beliefs.
So, it's like most things, buyer beware and know ahead what you're getting into.