In 30+ years of being gay there had been times when I engaged in sex with the b.f. of a friend. I never got to the point of snatching one away and living happily ever after. Anyone else ever do such a thing?
I had sex with the lover of a friend once when I first came out. He had come on to me in no uncertain terms, and I just assumed it was what everyone did. Those were the 1970s. Nevertheless, I assumed wrong. I don't really recommend having sex with anyone's bf. What comes around goes around. A man who has a bf and has sex with you anyhow is not likely to be particularly faithful.
I met a guy on the internet named Dana. He slept with a lot of married straight guys. From his discription he is a pretty good looking guy. And usually he is the one to get seduced, not the other way around. But he is willing to sleep with any man that wants to.