I know many would say the best man has a right to be there but it's your wedding too. If he's done something really nasty then he deserves to not go to the wedding. He was being deliberately hurtful, childish and immoral he sounds like a complete asswipe.
My friend phoned me up at 12am about 7 years ago when he was on his stag do. His wife had my number and he asked if she phoned up and asked he did not go to a stripper, which he did. I felt very uncomfortable with him expecting me to lie. So when she phoned I didn't answer and when he spoke to me the next time when not drunk I said for him to be honest to her, or we just simply wouldn't talk so he ended up being honest. It was terrible for him to put me in that situation.
I think my problem with this is that if you want to see other woman's lady bits then you should be single. Isn't the point of getting married wanting to just be with one person for the rest of your life? If you don't want that then have friends with benefits or be a free agent, it's not rocket science is it?
My DP's Dad tried to get him a stripper at his 18th but he was pretty disgusted by it, to be honest. I wasn't dating him at the time and he was still a virgin but still classed that as not nice so it doesn't always mean you have to be older and experienced to respect women in this way, you just have to be decent.
I know it would be my insecurity that would bother me the most about him doing that, as well as of course him wanting to see someone else as obviously that doesn't mean he wants to be with just me or at least not want to just see just me in that way. But at the end of the day though I would be upset that his best man was such a callous and inconsiderate twat. I wouldn't be too happy with having someone like that as a best man. I'd respect if my maid of honour did something distasteful too, so I wouldn't make one rule for me and one for him. I know he wouldn't do this though as he respects me, respects women, doesn't see what the big deal is and knows that although I am happy within myself in general, I still wouldn't feel comfortable with it.
I can't see why you'd want to use someone like that anyway. It is using someone for your satisfaction and not in the way you use a service like a sauna or even a human related service like a sports massage. This is invading someone else's privacy.
I can understand that you are really upset OP. My advice for what it is worth is to see how he behaves in regards to this in the next few days. If he seems remorseful and admits he did mess up, then I'd consider forgiving him but of course, not forgetting.
However if he says you are overreacting explain to him in your own words why you really aren't, so he can see it from his point of view. If he is still being callous and/or wont budge on the best man then I'd put the wedding back. Not cancel it, because I'm sure you can work through it, but at the moment it wouldn't be a happy time to get married and you want to get married at a time you are both going to be happy.
I hope things improve for you and he admits he was disrespectful and a bit of an idiot.
I hope you do have a lovely wedding. I am sure he is a lovely man and if he hasn't done anything like this before it seems nonsensical to break up with him for just this reason but at the end of the day you know what you are and are not willing to tolerate.