It's not a law. It's actually Mastercard.
Within the English sphere Mastercard has made the policy that they will not support Rape, Bestiality, Necrophilia, Pedophilia and Incest porno be it real or fictional.
They have applied pressure to Paypal to follow suit and in turn Patreon has complied.
Any attempts to reign in Steam... has been completely ignored. So you can fist a centaur that is also your mum on Steam. I am not exaggerating, that is literally the case.
Well if you can't get in the early stages for your project, then you kind of can't do it.