I don’t, personally. My ex did though, and it drove me fucking crazy. I could never understand why. Was he hiding something? If so, from me or his friends? I always got the impression that he felt he could have more fun if it was just him and them, which also drove me nuts. I felt like he held back if I was there, which sucked, because I would have liked to have fun with everyone. He just seemed like a different person around them. I don’t understand the behavior, because the entire point of being with someone is to incorporate them into the multiple facets of your life.
Personal time is still important, so guy/girl nights are perfectly acceptable and understandable. But not when it’s all the time.