They have monopoly power (or rather, duopoly power) because banking and payment processing are so highly regulated that there is no reasonable possibility of standing up an alternative, let alone one resistant to the same pressures that lead to events like this. The regulatory moat is so wide that they're effectively a government arm. We don't let the electricity and water companies refuse service to people over "conscience" because they are utilities with a government-sanctioned monopoly or oligopoly - it's the same principle, a principle upheld by the courts many times. For that matter, there are many private businesses required to provide their services to all comers just because what they do is too important. And to be clear, we're not going to have a deregulated banking sector, because history has shown us how badly that can go.
I'm not in favor of the age verification measures or general attitudes you mention, but there is a difference between gating access and defunding. Alcohol is also age-gated, yet it remains legal for those of age.
I give this reply more for others reading it than for you. You went from "I have read the bill, which is why I’m pretty sure it won’t protect NSFW" to saying that it's not an issue with the bill. So I no longer believe your weird selective libertarianism thing is being put forth in good faith, and I might not respond in future.