Your post implies that you didn't know you were touching another man's thighs. You should not ever reach blackout drunk. It is unhealthy, not fun (you can't remember the fun times), and but for your level of drunkenness you would not be inappropriately touching others.
Fortunately, you should be able to cease these actions with relative ease. If you can't control yourself while drunk, stop drinking so much while you are out. Set limits as to how many drinks you will have and then stick to those limits. Have a trusted friend or DD help you keep to those limits if-need-be.
Last, you should apologize to your fiancé for your recent behavior and explain to him the steps you are taking to avoid like situations in the future.
If you can fix this, OP, I think you two will be fine.
Hope this helps.
Hey, OP. Sorry you are dealing with this.
Maybe sharing my experiences will help you. I've felt like that before a couple of times. The vacation person: instant connection, devastated when parting ways.
During a cruise several years ago, I'd known someone for about 7 hours, if that. Southern gal. Really pretty, confident, and charming. She had a boyfriend, so of course we didn't do anything more than have drinks, dance, and talk. It was a blast. At the end of the night, we exchanged numbers and socials.
When the cruise was over and I was about to board the plane back to reality, I was legitimately moved to tears by the thought of not being with her. Months later, after she'd broken up with her boyfriend, she confessed she also felt a connection at that time. Nevertheless, within that day, the conversation fizzled out. She wasn't willing to be more, and I can't remember but I might not have been all in it either.
A similar situation occurred one other time while on vacation with a different woman, minus the boyfriend that time.
It seems I, like you, OP, become attached to that connection while on a worry-free, fun vacation. My educated guess is that we become attached to the idea of that person while in a stress-free, fun environment, and we are devastated when we have to return to reality. The seemingly strong feelings will likely fade soon.
Stay connected via socials, in case it wasn't just a vacation connection, but do not pursue her while she has a boyfriend.
Hope this helps.