I think it's more of a case that folk start to settle down and see less of their friends because they don't go out pubbing and clubbing, etc as much.
I have a group of friends that used to see each other all the time, but slowly we have all paired off and had kids. We still see each other and keep in touch, but it's not nearly as much as we used to.
I think it's just progression, whether we like it or not. It's not easy if you are the last one to settle down either.