Listen, I' not going to go all over-dramatic because you are clearly not wanting to hear it. Despite the fact I've worked in the sex industry and I can tell you with very few exceptions it's vile and full of exploitation (and not of men, the men have the choice of whether to partake or not, the workers - not always or even often). Maybe it was peer pressure, but I couldn't respect someone with so little backbone. Maybe his true character is coming out now. I don't know or profess to know about your relationship.
I will say one thing though - use this as an opportunity to define your boundaries with your P now. Let him know that under no circumstances will you tolerate him setting foot in one of those places again. Someone upthread made a good point in suggesting she would say that if he wanted to partake again to get on with his life without her. I'd say something like that, and also maybe educate him on a few home truths about lap-dancing. I am telling you that touching DOES happen in private booths. Sometimes more. It's not EVERY club but it's not uncommon. The majority of the dancers come to hate men. I have never been a dancer, but have known lots of dancers as the prostitution and lap-dancing industries go hand in hand and are very connected.
Define your boundaries with him now, and if he disrespects you by doing it again (however much "peer pressure" he gets) or tries to tell you you are wrong/ a prude for not wanting your partner to at the very least sexually objectify and ogle another woman's body, then you will see his true colours then.
It would maybe be a different story if you were OK with it, but clearly you are not, so you have to let this be known and give him a chance to respect you more than a quick sexual moment of objectification.