They are the same sex. They cannot reproduce.

If a cis man and cis woman are in a relationship, but the woman is infertile, are they not in a heterosexual relationship?

I'm not denying the sex of the people involved however, that is not how I see sexuality. It's not based on chromosomes, to me, and technically by definition either. I still feel invalidated because you and I see sexuality on different planes. If a cis man is dating a trans woman with a penis, I believe they are in a hetero relationship while you believe it to be a homo relationship. If the woman gets surgery, it's still a hetero relationship to me whereas in your eyes, it would still be a homosexual relationship. I can't make sense of this.

If you were born blonde but dye your hair black, your argument is that you are forever and ever blonde. I don't believe so. Their hair is now black and i'm not going to insist that their hair blonde when it's clearly not. Obviously like you said, labels don't matter, but it's the insistance. I know you don't walk around screaming these things at others and yes you respect people's identities to their faces but do you do the same behind their backs?