You didn't blindside her because he had her permission to start a relationship with you. Was it supposed to not include kissing?
I am wondering, did she witness the kissing? That could have triggered her in some way if he kissed you somehow differently, or more, than he kisses her. Which isn't your fault at all, of course, but you do seem to be wondering how this happened.
Regardless, you don't want to be with a man who caves to a groundless veto because his primary throws a tantrum. He may have other partners, but who knows how many other women they have already done this to?
I was once on the receiving end of a veto when the primary partner got jealous. I had been with him for a year, but we had only recently fallen in love. I was stupid, I hung in there, tried to pacify her, begged him to stick up for me. All I did was prolong the inevitable, getting dumped by him. She was OK with us staying "friends" at least online, and I still loved him, so we tried that. I spent almost a year IMing with him and seeing him for a couple of teary in-person conversations too, and trying to persuade him that he should assert his right to continue the relationship with me. He told me repeatedly that he still cared, and missed me, but he wouldn't do a thing to change the situation, just kept giving me false hope.
Well, that guy was not my friend. He eventually showed his true colors to me not too long ago, he is a petty, judgmental, hypocritical piece of crap, and I was finally able to walk away from him decisively.
My point is, your guy can't possibly be a good guy either, if he treats people who care about him like disposable garbage. I wish I had figured that out sooner, I would have spared myself a whole lot of pain.
Have you considered telling his other partners what happened to you, do you know them at all? If I was dating this guy, I would sure want to know how my relationship could evaporate on his wife's whim. People like him need a warning label.