I don't have kids. I am not infertile, and though I have always wanted children I didn't. It's sometimes sad, but on the other hand it's not something that I regret as I don't think I would have been able to bring up a child as a single mother. Other people can, and I admire them enormously, but I couldn't have.
Kelli gave you some great insight. On the days that I can't do an activity full of kids, I feign illness or arrange to have to do something else that day. But not including me in my friends' activities especially those involving the kids is also hurtful. *I* need to make the decision for myself each time there is an event of some kind. It's really not right if people make the decision for me because they want to spare my feelings.