I was told the same thing you were. It's overly protective, IMO, but has a strong basis and truth to it. It's as if you are still engaging in a "single" life by going out with single people when it's suppose to be you and DH with other like-minded couples.
I think it has to do with the thought proccess and actions of being single. When single, you don't have to consider anyone but yourself in your decisions. When you are married, you are suppose to be "one" and your actions and what you allow into your thought process can slowly have an effect on your husband/marriage.
I don't go out but once a month w/o DH and that's to meet with my childhood friends (all single). I'm pretty careful at what I talk about and where our conversations go during our "ladies night out" dinner so that our meetings are a personal time for me to spend with friends instead of trying to be "one of the girls" again. I've scared them all into remaining single.