I agree with you Lard my husband and I have both found new friendship groups (both individually and as a couple) through having children.
We're not friends with these people because we all have children we just happened to meet some really nice, interesting people through our NCT group, Mother and Baby groups, nursery and school.
We do live in a small town where you tend to bump into each other at the shops, library, park etc so I think that probably helps foster friendships.
We've both maintained our friendship groups from before we had kids but there's room for both.