But many posters openly say they are anti trans and deny the possibility of a person being assigned the wrong gender at birth.
Do you know what you are saying here, or are you just parroting what you have heard?
Firstly, no one is assigned a gender at birth.
Transgender people say that gender is an inner feeling. Once you are old enough to understand that humans bodies have two different sexes and that certain stereotypes are attached to those sexes via clothing, colours and other external fabrications, you may feel that your gender does not match your sex. Not everyone agrees with this but that is how it is explained.
It is therefore impossible for a doctor or midwife to assign a gender. The newborn itself is not capable of that sort of thought process.
So the medical staff and parents observe the baby's sex.
Therefore, denying the possibility of being assigned the 'wrong gender' at birth is not anti-trans, it's a simple fact.
I get that you may not have thought about this and realised how illogical a statement it is though and that's the main problem really.