The night everything ended with my exWBF, he called me a slut. I did have a lot of experience, but it was when I was single, and I had never lied to or misled anyone, ever. I believe he said that to me a sentence or two after telling me he was intending to take home and fuck the woman he was talking to at the bar when I walked in and found him there.
So, it is all projection. He can't face what he is, so he turns it around on you.
It's also about insecurity. He never had the confidence or self-assurance to brush off what happened during your split, which any mature and well-adjusted person could absolutely do. that lack of confidence eats at him. But instead of facing that it stems from his own self-esteem issues (which would require working on them), he decides it's about you being a bad, unfaithful person. The energy that should go into healing himself, which is difficult, goes into attacking you, which is easy and makes him feel powerful and justified.
It makes you crazy, I know. There's this desire to make them get it, to understand that they are wrong, and unfair, and have everything ass-backwards.
Unfortunately they aren't able to get it, or if they do, we will never know.