@Bad_Banner to them it's not really about consent, it's about the perceived harm done to them and they don't believe any kid who thinks they are trans will be harmed by transitioning. Or at least, they think it is a small enough amount of kids that will be harmed that they don't care.
That's looking at it in about as kindly as I can. For a lot of them I'm sure they're just retarded and think someone they don't like fucking/grooming children is bad but themselves or someone they do like doing it is good (like that fucking Mr beast dude).