You don't blame them. We were all thought to see spinsters as incomplete gender races. I'm having a similar conversation with a guy on a comment I made on a post about how society thinks women are incomplete without a spouse. Like marriage is the ultimate goal for every living being.
It's that ingrained patriarchal doctrine set to undermine women. What makes me sad is that our generation is still reinforcing this toxic doctrine. If your nieces think this way then their mother or father must have said something (about your situation) not being normal.
It's sickening. I mean how do we expect gender equality and respect for women if we are marginalizing some women due to their disability, fertility or whatever differences they have?
What we count as normal might be abnormal to another, yet we should respect people equally because we are all human and we deserve to be seen, heard and respected.