It sounds like you have more serious issues to deal with than him going to see some lapdancers. Whether we women like it or not, men are easily turned on by what they see than women, that doesn't make them cheats or pervs though, they're just luckier than we are! It's just as well for men that we are more turned on by personality, humour and kind gestures as few of them are worth looking at in 'that' way anyway, lol!
If I were in your situation he would be the last person to know I objected to his going. Not being disrespectful to men but it's like when you were a kid and someone told you you couldn't do something, you wanted to do it all the more and would then sneak about to achieve your goal.
As many have already said, tell him you've had a word with your girlfriends and you've had a change of heart, they've convinced you that strippers are hot and you can't wait to see your fair share of fabulously toned, tanned, XXX-large bodies too, woohoo! Men are a lot more paranoid about their body image than we realise, you won't change his mind about his stag night, and nor should you, but you may plant the same seeds of insecurity you have about saving those thoughts for when you most need them, lol!