Obviously I feel that there should be more representations of mothers and families in genre fiction; I was on a panel about exactly that at Wiscon. Genre's skewed towards 20somethings in leather pants. I am a 30something in leather pants with a teenage daughter, and I still have adventures!
But I myself have never felt marginalized because I'm a mother.
I was the first of my friends to have a child. I didn't know anyone my age with a kid my daughter's age til I met irana when I moved to Atlanta. I didn't have a group of kids Elayna's age with parents roughly around my age until I moved to Boston (range is about 35-50 here).
But I never felt marginalized, even when I was alone.
I've been a mother my entire adult life; I got pregnant with Elayna when I was 20. She was due before I could legally drink. So she is not something I view as separate in my life, if that makes sense. I was attending cons with her strapped to my chest in a sling (Hal Clement took a picture of me reading to baby Elayna at Tropicon); I brought friends to panels with me so we could take turns taking toddler Elayna outside if she got restless. I brought little Elayna to DragonCon babysitting; teenage Elayna roams the halls at Arisia, and is on panels right alongside the grownups.
I have not noticed anyone having a problem with this.
I can't really extricate "being a mom" from anything else about me; it's like being short or being a writer or wearing glasses. Incontrovertible parts of me. I'm a mom in fandom. I'm a short person in fandom. No big. It just is. If anything, I have to think about being short at cons more than I have to think about Being a Mom; at this age, I can turn Elayna loose with a list of what panels we're on and a list of rendezvous times and places and twenty bucks for the dealers' room, whereas my height (or lack thereof) means that I often have to get assistance from a tall friend to navigate through crowded lobbies (because all I see is a wall of torsos).
If anything, I find that people dig the fact that I'm a mom. zarhooie was only the first to say "I want you to be my mom, too!"
So. Y'know. Just some thoughts. *shrug*