UPDATE:

First up, I'm really grateful that people like you and places like this sub exist to help and inform people. I'm in a much better place emotionally and don't feel so lost and confused anymore.

Just to clarify, he didn't 'get caught' as such. He told me while we were having sex and that's how I found out. And then he immediately stopped. So if he was planning to take advantage and trick me, he is clearly terrible at it.

Well, today I reminded him that I was still upset. This time I straight up asked him why he did it and he struggled to answer. He eventually said he got caught up in the moment, didn't think about what he was doing and that he realises it was wrong. It didn't sound like he was making excuses since he didn't say any of this until I specifically asked for a reason. Also he hasn't tried to minimize what happened or gaslight - and isn't melodramatically grovelling or lovebombing me with affection like my abusive ex used to.

I laid out some ground rules for the future which he readily agreed to. I repeated that if anything like this happened again I would leave, and he said he understood and promised me nothing like it would happen again. He keeps apologising to me.

Since this is the first time in 5 years together that he has done anything like this I'm willing to accept it could have been a one time selfish mistake, but I'm going to stay vigilant and am completely prepared to follow through and leave if this becomes a pattern.

Even just a few years ago, the old me would have quashed those emotions, pretended everything was fine to avoid confrontation and made all of the excuses to cover it up. So thank you all for your support and advice. Hopefully I won't have to post any follow ups.