Tell your woman why you need to go out and party. Relationships are built on communication, and you'll never find a better example of it than this situation. She can be more indulgent than you think, so why not give it a shot?
And while you're on an honesty streak, why not tell her the gist of your evening? Females have the unfortunate tendency to become insecure about the little things. You know she gets hysterical about the mailman not being on time, so how do you think she'll react when she knows you're about to embark on a journey to the dreadful depths of human degeneracy? Do yourself a favor and tell her most of the things you and your buddies will be doing. You can leave the spicy details out if you think no serious damage will ensue.
I know that boys' night out can be extremely entertaining, but try to keep yourself from doing it too often. I'm aware that some of you may want my head on a platter for uttering such heinous words, but I only have your best interests at heart. Going out more frequently than your girl can brand you a selfish deadbeat. She's most likely working as hard as you are and deserves as many good times as you do. So, the moment you're starting to have more fun than her, your stock plummets. Moderation rules.