So yeah, being mad at Valve is stupid, people need to be mad st MC and Visa and probably also PayPal.
Being mad at Valve is shooting the messenger.
Fortunately the petition is at least correctly aimed at the payment processors.
But also...
If MC and Visa won't budge on their positions, well, if Valve then makes an alt payment system for adult only games...
MC and Visa go, oh, hey, you're violating our guidelines, we no longer support Valve/Steam, now no one can buy any game.
This is a MAD situation, Valve would have to come up with a comprehensive payment processing system for everything, in secret, and then deploy it all at once.