Here's how I see it: you're not the one who made the vows, thus fidelity to the wife is his responsibility, not yours. That said, I think dalliances with married "straight" men are train wrecks waiting to happen. First of all, I don't really mesh with a world where men are "straight" and still want to have sex with men. I can eat meat and call myself a vegetarian, but actions speak louder than words. I think there are straight men and gay/bi/queer men who want to be perceived as straight because it benefits them (at least in some ways, to their minds--I certainly thought authenticity and self-care was the better choice). I've had a number of experiences with these guys, from a frat boy I made the mistake of getting involved with back in college to dating apps as an adult/professional in which married "straight" men have proposutioned me, i.e. for "sloppy BJs" (when I asked one how cheating on his wife with a man made him straight, and whether it was difficult living a double life, he blocked me--these guys don't like being checked). Recently, and painfully, I had a friend I considered close treat me weirdly for about a year and cause some serious anxiety. He stared at me nearly constantly, like turning his head staring, to such an extent that others noticed. He touched me in awkward (and visible) ways when he touched no one else and made it a point to mention how he didn't like touching. He talked about me so much to others that even my straight guy friends were coming to me saying, "What's up with this dude?" But the next minute, he was tokenizing me--"we could go to a gay bar, but we'd need a fat guy to get in" (when I was the only gay guy in the room), sending me pics via text of gay sex in books he was reading, constantly asking me questions about my sexuality, making my sexuality the butt of his jokes. He'd let slip comments about divorce or how people shouldn't marry too young or how much he drank. Class and work eventually became an uncomfortable space for me, because I knew he was either going to be too affectionate or too dehumanizing. Finally, I confronted him and he not only denied any weirdness, but placed the blame for all of it squarely on my shoulders (because the gay dude is always guilty). This is not uncommon. I know so many people with similar experiences. If you are capable of cleanly dividing sex and love and you want to scratch that itch, go for it. But my gut says these guys in their denial and sneakiness and selfishness and infidelity and dishonesty are completely unstable. Why put yourself at risk of catching these guys' shame? What about shame and self-loathing is so attractive? Is this internalized hetero supremacy in the heads of gay men that says "straight is more desirable?" Take care of yourself. My two cents: even if sleeping with a "straight" married dude isn't unethical for the third party, you deserve more than some guy in an identity crisis. A guy who knows who he is and owns it is far sexier.