The crucial difference is that the affirmation only approach to children who express gender dysphoria puts them on a pathway to a lifetime of surgery and medication, when in fact there's a lot of evidence to suggest that while some may grow up into adults who continue to express gender dysphoria, and for whom transitioning medically may be appropriate, most in fact turn out tohave other issues going on. For instance, they may be autistic (which is going to make you struggle with social rules around gender expression, because social rules around gender expression are on the whole pretty arbitrary, opressive to women in particular, and make no bloody sense at all, which makes them pretty hard for someone with autism to understand). Some may turn out to be gay, and were in fact struggling with being same-sex attracted in a heteronormative world. Some (particularly girls) may simply be struggling with an increasingly pornified, sexually violent culture in which to be female is seen as being fair game for men to sexually objectify you.
With the rise of affirmation only therapy over the last decade, we're already beginning to see a wave of young women in their early twenties who are detransitioning, often having caused irreversible changes to their bodies (mastectomies, voice breaking, etc.).
It's also interesting that in the narratives of parents who are cheerleading their children's transitions the same tropes come up over and over again: "we knew something was wrong when he/she started playing with dolls/trucks." There is definitely an aspect, among some parents of children with gender dysphoria, of "transing away the gay." That's why I'm far from sure it's the wonderfully forward-thinking, progressive movement some on this thread seem so sure it is.
For this reason, unquestioning acceptance of a child transitioning is a totally different thing from unquestioning acceptance of their race or sexuality (and IMO it can in some instance be borderline racist and homophobic to draw this false parallel).