I was with a cheater for two and a half years. He was my first poly relationship and I was very much in love with him.
What I knew for most of our relationship was that his marriage was very troubled, he and his wife rarely slept together their last few years together, and that he had cheated on her with another woman. I persuaded myself that it wasn't so bad, because he'd really loved the other woman, and love makes things so much less sleazy, right? It never even occurred to me, in the beginning, that the very act of cheating with even one person might have made him less trustworthy as a partner.
Boyfriend left his wife right around when he met me, after having first tried to negotiate a poly relationship with the wife. (That didn't work, the wife wanted other lovers but wouldn't let him have any of his own, or so he told me, and he left her at least partly over that issue, although the marriage was troubled in other ways too.)
Well, I believe my boyfriend didn't cheat on me, at least. He had three other partners while we were together, and I at least knew about all three, and knew one in person pretty well too in person. But people who lie and get away with it, they get good at it, and I believe that many of those will just continue to lie if it suits their ends.
After two years together, my love's other poly partner, who had started out as my friend, had grown very obviously hostile toward me and had even asked him to dump me. I had gotten upset about that, sought advice in an online forum of people I trusted about mending metamour fences, and had gotten some good advice there. I never got to use it because said metamour saw my post, was convinced that everyone in the forum would know I was talking about her (even though there was nothing in the post or on that site that could connect the two of us, and certainly no names were ever mentioned), and, as far as I can piece together, got our boyfriend to create a web of lies that were designed to make me shut up and never post again about our relationship. Most of what he told me was so preposterous that a child would not have believed it, plus his story kept changing anyway. I called him on all of it, demanded the truth, and he dumped me in a one-paragraph email and never talked to me again. Even now, a year later,when I see him in a social situation, he won't even say hello.
Shortly before he ended the relationship, boyfriend confessed to me that he had not only cheated on his wife with the woman I knew about. He apparently was on adultfriendfinder for the better part of a decade, banging any woman who was willing, without his wife's knowledge or consent. I hadn't known any of that. If I had, I don't think I would have dated him in the first place.
At this point, I don't think I could ever trust a person enough to have feelings for them, if I knew they had a history of cheating on a wife or a live-in partner. I look back on my relationship with boyfriend and it feels like the entire thing was all lies. I regret that I ever met him, and if I could slap a warning label on him for all the women in the poly community, I would. It's probably unnecessary though, because I don't think my former metamour will ever let him have anything that goes beyond casual sex with anyone else. She has a long history of cheating on her former spouse too, surprise surprise.