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Give her an ultimatum, in a jokingly manner, like..
You: "Are you sure this is what you want? No regrets? Dont Blame me if i end up getting drunk with friends too and something happens, something like a girl getting lucky with me haha..

".
***Remember, the tone and how you say it should be with a smile , like a joke.
Then explain to her that, she is in a relationship, and that what she want to do, are for single people only. Couples talk, reminder: Keep the conversation FUN! as this will trigger her emotions to comply.(most probably). But, if she still wants to go out and get drunk. do the same , and make a random girl lucky!
Cheers!
Don't do this. Ultimatums are expressions of powerlessness, and if you do it in a half-joking manner, she'll respect you even less.
Instead, realize that you can only ultimately control yourself.
While she's out drinking with her friends, why not go out drinking with yours?
Better than sitting at home wondering what she's doing, is it not?
Ultimately, the best way to prevent her from cheating is to have her respect you, which you can do by living your own life and not worrying about things like what people are doing with their free time.