Mother of one teen here. I do not feel like less of a mother, however, I have been made to feel like one since my child was like...four. I have pets, a significant career that I juggled working school hours from home, and despite this, the other mums absolutely looked down on me. I had more than one stay at home mother of two or three children ask me to do favors like childcare or school drop offs because they perceived me as having more time. Some even had full-time nannies, while I was "doing it all."
I felt excluded from a lot of conversations, cliques, and eventually, friendships altogether. I felt like a lot of people would make little digs about how they were so happy their children had each other, etc. Every time someone else got pregnant, I absolutely felt more shunned.
No one knew why I only had one child. No one knew if I would have loved another.
I ended up keeping mostly to myself for most of my child's childhood as a result. The only silver lining was that my child eventually got into a sport with most only children, so I felt more understood there.
But yes. I absolutely felt judged and even now, it's still a sore spot.