I'm just going to have a boys only night
Why are you not inviting your girlfriend?
Because I want to do something naughty.
Hiding out from your girlfriend on a "boys night out" and keeping her out could be wrong, depending on what you mean by "naughty."
She should understand that I'm allowed to do what I want sometimes.
I agree, you need to have a live outside of your relationship, as long as the "naughty" isn't illegal or too transgressive.