my residency is about 50% married and of those married, roughly 50% have kids. Still, there are plenty of people within and outside of residency to go out with. I think the key is being in a hospital with lots of other residents in a large city with lots of other young people. Boston is crawling with college/grad-school-aged people. Chicago and New York are also good (I'm in Chicago). Rochester, MN, on the other hand... probably not as good.
Probably just as important is feeling out (not up) the residents on interview day as to how they all get along and what the social life is like.