I understand that people aren't comfortable bringing Daniel's own sex life into the debate. But by stating that sex doesn't matter and gender does, Daniel himself is making a judgement. That school of thinking is used to pressure trans widows to stay with their husbands through transition, it's used to bully lesbians, and it's used to undermine women's sex based rights. Daniel deserves to be held to account for that.
I'd suggest asking him the hard questions.
"Daniel, I am a lesbian and only attracted to natal women. Would you defend my rights are fiercely as you defend those of transgender people? Why / why not?"
"Daniel, I am a man and don't feel I should have to learn to be attracted to penis in order to be a proper ally. Do you accept that there are important differences between women and transwomen, or do you think it's hateful for every human not be bisexual?"
"Dear Daniel, why did you as a man feel it was your place to speak on a feminist issue, let alone speak over a woman on it?"
"Dear Dan, my Harry Potter movies have gone straight in the bin. I now consider JK's books to be the only true Harry Potter content. Thank you for helping me see that if I didn't like your opinions, I could simply cancel you and pretend you don't exist."
"Dear Daniel, what is your position on the troubling side effects associated with off-label use of Lupron? Do you believe parents should stay silent about this out of fear of being called transphobic? Or should they look for less harmful ways to manage their children's dysphoria? Would you be willing to use your considerable platform to support parents who might want to sue? Why / why not?"
And so on. Ask him calm, rational questions about the knottier aspects of the transgender issue, and let him hang himself with his own words.