When my ex finally admitted he'd cheated, he said they got into bed and then couldn't perform (his excuse - he thought of me and then couldn't do it
)
It was totally bollocks, which he did finally admit.
He's probably lying about not being able to perform - unless this is a usual factor of his sex life - is it?
So many cheaters when caught, do this drip drip thing, where they only admit to the things they think they have to, but more of it will probably come out over time.
But even if he isn't lying, it's a distraction. The fact that he got into bed with her and would have done it if he wasn't so drunk is irrelevant.
He intended to cheat on you and did - even if no actual PIV sex happened.
That's the starting point for the next stage.
You also need to do some digging. He "bumped into" his old work colleague, did he? How did that happen? Are you sure it wasn't arranged? Seems like an awfully convenient coincidence.
I would ask him to hand over his phone, email, Facebook and bank statements immediately for you to look at. Don't give him time to delete stuff. If he won't he's hiding more, don't accept any excuses.
If he complains about you not trusting him, let him know that he's acting exactly like cheats who are caught out do - he's sticking to the script word for word in fact (he bumped into her, they didn't actually have PIV sex), and that if he to have any chance of you ever trusting him on any level again he will do this for you.
Even if you do want the relationship to survive - perhaps especially if you want the relationship to survive, you need to ask him to leave so you can think about what to do next.