Why are you left out of a lot of circles and social functions? One of my best friends has a kid now (I don't have kids) but he still finds time to get away from time to time thanks to a sitter and we still hang out from time to time. Even when he'd have birthday parties for his kid and I'd go to those, even though 75 percent of the people there had kids themselves. He'd still have gatherings for events (like the Super Bowl, Kentucky Derby, etc) at his house... kids around, but I'd still go and have a good time there.
Do your friends just do things that center around their kids and assume you want nothing to do with it? Do your friends never get alone time to hang out? I'm genuinely curious.