Veruka - I can very much relate to what you have said about your circle of friends. I also am the only childless, unmarried friend in my crew and I have been friends with this lot since Junior Academy. Most of the events that I go to with them involve kids, but every now and then, they do something without their kids just to satisfy little old me.
Normally I have to plan that certain something. We are having a big evening out later this month that I have planned and it should be alot of fun.
I don't really have that much of problem with activities involving their kids --- their acceptance of me being happily unmarried is another issue
I have made some new friends of late that are in the same boat as me, but nothing would ever replace my friendships with my original crew.