How are Space Marines humans...

How are Space Marines humans? They clearly are fucking mutants just not the warp type I would like to remind you that humans have only one heart, can't spit acid and don't live for centuries.

They round off to human in the larger context. Remember, you're fighting fungus that shoots you with mind bullets.

Well when a man loves the Emperor and they put a dozen different extra organs and growth serums into him..

Nice bait.

Oh but psykers who look way more human suddenly are mutants right?

they're post-humans

you tolerate them because they were made by your god and share his blood

That faggot vegetable on golden toilet wasn't human either.

They channel warp. It's less normal than being a big guy with parts of primarch bioengineered in

They're still genetically and born human but augmented to the point where they can fuck up the enemies of Man.
Also you probably dont wanna ostracize the supermen who keep the aliens from felating your asshole

It's a very, very useful and very, very mighty vegetable, so...

They're just humans with lots of augmentations. Same as the techpriests, just more organic augments than mechanical ones. They still retain the human form.