It's less common than it used to be. Most dorms at my undergraduate school were co-ed. I think the two single-sex buildings were holdovers from its military-school days. My current institution is in the process of building new dorms to replace those built in the 1960s. The old ones were single-sex, and I believe the new ones will be co-ed. I think most schools are going co-ed because the students expect it, and it provides more flexible use of the space.