It's an attempt to dismantled women as a class. So instead of being women, some of us are uterus havers, or pregnant people.
We won't all experience pregnancy, infertility, unwanted pregnancy and termination of pregnancy, but we all have the potential to. And thats what links us all. We don't benefit from subdividing ourselves up into different classes.
It must be frustrating for trans men to feel excluded, but instead saying 'uterus havers' it's clearer to say 'women and trans men' where appropriate. But is that really the issue? Do trans men really believe that women fighting for the right to terminate a pregnancy are purposefully excluding them from the same right?
I think the pressure is really coming from male people who want to remove the link between 'women' and female biology.