A lot of my friends do that, I say about 50/50. They just disappear when they are in a relationship, resurfaces during Christmas pub crawl and may be a stag do. One was so bad, when he was single he was out a lot. When he was with his gf/no wife, we used to call him to come out, he says he would. but he just not bother to turn up and not bother to call. We gave up on him in the end, lately his marriage is a bit iffy and he's come out more, then the relationship got better and he disappeared again.
Another friend, pretty much the casanova of us lot, go through about 4 girlfriends a year on average, and only really see him when he is between girls, then disappears when he has one.