Pronouns are not based on biology.

Either pronouns are based in biology (which they are), or they're based in misogynistic stereotypes that a woman is someone with a pink brain, someone who enjoys misogynistic oppression, someone who likes to wear "women's" clothes etc. etc. The alternative to pronouns rooted in biology is pronouns rooted in defining men and women according to utter misogyny, and that's why radical feminists have an issue with them and don't want to use them.

Also, pronouns have always been based in biology. Trans people themselves are extremely vocal about how oppressive this societal norm is. So claiming that actually, everyone already uses pronouns based in genderfeels and trans people have nothing to complain about feels like intentionally playing dumb, since that is one of the biggest points of contention and confusion between trans activists and the majority of people.

when people use "she" for me they are validating my identity. Why is different about trans people?

People do not use "she" to validate your identity. They use "she" because it is an objective fact that you are an adult human female. Feelings, politeness, niceness, wishful thinking, your zodiac sign etc. have nothing to do with it. If I call you a vertebrate, or a mammal, I am saying that because that's what the reality is, rather than because that's what you want to hear.

If we're talking politeness, trans activists could start by not lying about and supporting the oppression that women endure as a result of their sex, and by not erasing, downplaying and twisting the language needed for feminist discourse because it hurts their feelings.

Buck is a man. He has a beard, deep voice and male levels of testosterone.

That's not what being male is. There are plenty of non-trans women who have deep voices, facial hair and abnormal testosterone levels. There are also plenty of non-trans men with no facial hair, high voices and significantly lower testosterone levels. Science has long since accounted for this. These are not people of the opposite sex, or third sex or whatever, they're either just a natural variance of male or female biology, or they have a disorder of sexual development. If anyone with male testosterone levels was automatically male, it wouldn't be possible to define a disorder where a male has male hormone levels, or any disorder defined by the sexual biological function malfunctioning. That's why no-one would bat an eye at a boy claiming he needs to see the doctor because his period didn't come in, regardless of whether that boy identified as a girl, a menstruator or a dragon.

Buck's body is a woman's body, morphed through years of maintaining an artificial hormonal imbalance (which has also lead to debilitating health issues). This is completely different from a male body that is male because it's MALE. This is a masculinised female body, not a male body, just as a castrated male with an inverted penis has nothing to do with an actual female. It is simply not scientifically possible to change one's sex in any way. The only thing you can do is artificially induce a hormonal disorder.

Even if you don't believe in this language you can still return the same respect that Buck gives to you.

Acknowledging reality is not something you do out of "niceness" or "respect". I will not pretend the Earth is only a few thousand years old just to be respectful to Creationists, nor do I expect anyone to acknowledge the actual age of the Earth solely out of politeness and to not get me offended, but rather because this is an objective, measurable fact.