Some people believe in God. Some people are alive today because they found God, and their faith has stopped them from killing themselves. Doesn't mean God exists, and it certainly doesn't mean I should have to believe he exists. I respect the right of others to believe in God, but I won't pretend he exists just to keep them happy.
If trans women want to believe they are women, that's their right. But there's no scientific evidence to back up that belief, so I'm not going to subscribe to it.
What do I believe?
I believe that the big enemy here is male violence, because that's why women are worried about self ID. But the trans movement isn't really talking about that, is it? That's because it's a lot easier to tell women off for being transphobic than it is to stop men being violent. If male-bodied trans women's access to women's spaces is dependent on preventing male violence, it won't happen will it? So the issue is ignored.
I also believe that the important concept of sex segregation has been undermined because of the conflation of gender and biological sex. I see Trump is now using that very same conflation as a way of removing trans rights. That is why it's dangerous to claim that gender (as defined by the trans movement) is equivalent to biological sex. The two are NOT the same thing. Gender that arises out of biological sex is different from gender as a feeling of what you are.
I also believe that we don't know what being transgender actually means. We have no scientific understanding of the mechanism behind gender dysphoria. We don't know if it counts as a disorder or not. We don't know if everyone who has those feelings is having them for the same reason. Psychiatry is not a mature science, and the field often makes sweeping decisions that turn out to be wrong. Autism, for example, was until very recently considered to be an extreme version of the male brain (and that's considering there isn't even good evidence to say there's such a thing as the male brain).
I also believe that it's unfair to let trans women compete in competitions for biological women, and for trans women to be allowed to take advantage of shortlists reserved for biological women. I support them having their own competitions and their own shortlists, but sporting events exist because women have female biology, and women-only shortlists exist to counteract misogyny and not transhpobia.
I also believe that the reason the problematic aspects of Self ID and "transwomen are women" are being overlooked is because society is still Patriarchal. The concerns of women are being dismissed in the same way that the concerns of men are dismissed when we debate men-only golf clubs. That is not right. A women's refuge is not a golf club. The reason some women want penis-free changing rooms is not the same reason some men want women-free golf courses. It's an example of unconscious misogyny when the concerns of women who have suffered rape or abuse at the hands of men are downplayed in this way.
So, there you go OP. Is it transphobic to point out why the female biology of women is so important to the definition of "woman", considering we live in a misogynistic society where the protections we give women are necessary because they have a female biology?