You can't deny people's existence and still expect a meaningful discussion or cooperation.
Nobody is denying your existence. We're pointing out that your body is male, which would be impossible if you didn't exist, as, indeed, would this conversation. To be blunt, it seems rather as though we're saying things you don't want to hear, so you're falling back on that daft slogan because you don't, actually, have better answers.
A starting point would be to accept that transgender men and women are the gender/sex that they tell you they are.
That would be your starting point, yes. It's not ours. Just as we wouldn't dream of insisting that you accept our view on this, you have absolutely no right to insist that we accept yours. We think sex is real, that it matters, and that women need single sex spaces for our own dignity, privacy and safety. We also accept that trans people have every right to live in safety, happiness and dignity. You have every right to make good choices for your life, and in the way that best serves you. But when you walk in on us when we are naked, your choices have suddenly become our business, because at that point, two sets of rights come into play. And you don't have a right to be with us when we're naked without our consent.
You think biology is irrelevant. I'm going to assume, given your feminist values, that you think gender stereotypes are too. So can you please define what a woman is? What exactly do you mean by 'woman', if you're eliminating biological sex from the picture?