Situational ethics is a wonderful thing, huh.
Let's turn this sideways just a bit. What if your SO felt the same way and was getting some on the side without your knowledge? Hey, you didn't know so you weren't hurt at all by it. Right?
I don't know if humans are the only animals that can rationalize their behavior because a situation benefits them, but we certainly seem to take advantage of it when the occasion calls for it.
If you are in a committed relationship where you have sworn to
forsake all others, and you find that the burden is too much to bear then you have broken your vows.
Best thing for him to do is to allow his wife to do the same. If she's not willing then say the rules have changed, and you can stay or you can go, but this is how it's going to be from now on. That sounds as compassionate as one can be in that situation.
Well, he's the one that's married, not me. Yeah, I've heard that before. I have actually unwittingly been the other woman in a relationship with a married man, and it sickened me to learn that.
Still, you want to know if you're in the wrong. That I cannot answer. I'm not your conscience, but you are contributing to the decline of an existing relationship. But I'll go so far to say that if it's not you then it'll be someone else.
I'm not an expert in situational ethics because it's a moving target.
Your best bet is to go find a single guy with a fat weenie to take care of. Why would you want to be entangled in a web if you don't have to?
For reference:
https://www.lpsg.com/xfa-blog-entry/bad-dates-2.3644/