Despite what all these other macho fucks told you, you may have a legit concern. What does going out mean? Dinner and some drinks or a club? How often is this "going out"?
There is nothing good coming out of married couples hanging out at a club with their friends so if that is where she is hanging out: 1) don't try to one up her 2) tell her you don't approve.
If she is just going out for dinner and martinis, you got to let that go as she should if you are going fishing, hunting, or hanging with the bros.
I agree with Dr. Bill, despite my comments above. I'd definitely be pissed if my wife was clubbing every week. But I'm glad she goes out for dinner and drinks every once in a while.