To me, Terrigenesis was never about "correcting imperfections, but rather embracing change in all it's forms and unlocking one's true potential, being who you truly are and being all who you can be. Surely no mutant could object to that..?

That's not what he seems to be objecting to though. It's the notion that young children are being indoctorinated with an ideal that ONLY by rewriting their DNA can they truly be accepted or be considered "whole", and I can well imagine a mutant objecting to that.

As I said, what happens if someone doesn't want to be Terragenised (Karnak's parents made that choice for him, it wasn't his decision) If we'd seen the candidates being asked with a no-shame drop out option, (maybe a private ceremony, rather than the ultimate peer pressure moment of everything happening in front of all the already mutated Inhuman crowds AND the Royal Family) (and that may have happened elsewhere) I think it would come across better.

The religious indoctrination comparison is an interesting one, though the choice being made is so... extreme and one way, that I don't think there are many comparable situations.

And someone who has hit puberty is still not legally recognised as being anything other than a child.

Yes, I think that Kurt reacted through anger, and a few questions would have made things smoother (though the same could be said about 98% of superhero bust ups) but he thought he saw children being put through something he found unthinkable, I'll give him a pass for that.