I view strip clubs in the same way as I do looking at porn: sure, none of us love the idea of our partners doing it but it’s just a fantasy and not cheating at all. These girls are literally being paid to stand around and dance naked while men pay to stare at them. It’s purely a transactional thing and no one is going to fuck anyone. I mean, there are places where you can pay extra for that kind of stuff but for the most part it’s just voyeurism.

Of course, you are entitled to your feelings but given that you have never discussed it and therefore never established it as a boundary, I don’t think you can really be too upset here. Especially since it’s his birthday and he was dragged there by his friends. He also didn’t try to lie his way out of it, which is a good thing.

If you want to establish it as a boundary, now is the time to do so but there’s no point in holding a grudge over something which already happened.