My (28f) partner (30m) kissed someone else years ago and never told me

My (28f) partner (30m) kissed someone else about 3 years ago and never told me until recently. This was a family member of his brother’s wife.

This was late in the night at a party (with friends and family) after we’d had an argument and I’d left.

His version of events: They were talking and sitting together in a group, she tried to kiss him at one point, then asked him to come with her, took him into a private room, jumped him and pulled him on top of her on the floor, tried to sleep with him. He says he resisted her kissing but might have kissed her back for a moment, was taken aback by the whole thing and left straight away.

Her version: He led her away into the room, they kissed, he picked her up and pushed her against the wall, then they left after a few minutes.

He says she’s lying. I feel like the truth might be somewhere in the middle.

He told me a few months ago and has apologised for it and admitted he was wrong for going away with her into the room, that he was curious. I feel broken that someone who one of their biggest good traits to me was their loyalty could be tempted like that.

I’m stuck on whether to just let this go as it happened so long ago, or whether the fact that he hid it for so long is reason enough to potentially leave (if I can never get over it?)

TL;DR My (28f) partner (30m) kissed someone else about 3 years ago and never told me until recently. Their version of events don’t match up and I don’t know who’s telling the truth and never will. Don’t know whether to move past it or something else?