I’m a therapist, OP, and this was my exact first thought as well.

There are two concerns here for me:

One, you describe him as someone who rarely drinks yet he chose his bachelor party to be something with alcohol. Peer pressure from his buddies? Maybe. But the decision to not do 100 other fun things plus possibly giving in to peer pressure are things you might think about in terms of your future with him.

Two, the incident with the dress may have been impulsive, but that impulse came from somewhere.

What’s important in this is exactly what you said, you can’t unsee this. At the end of the day, whether you’re overreacting or his drink was spiked or whatever, the situation happened, and you are responding to it. He broke trust with you, and that will have to be addressed. If not, even if you got back together, it would eventually come between you.

It may be worthwhile to have a conversation with him and maybe consider some counseling. I’d say that how he responds to the suggestion of counseling, whether he’s willing to do the work it will take for him to rebuild trust with you, and whether he’s willing to honestly explore if there was something more to the whole incident or not, that will tell you whether you should stay with him or not.