As another poster pointed out, it's not just the US, though with the US's economic dominance it's a huge factor. China and middle eastern nations don't like sexual content either. Basically, the governments in these places want you to concentrate on the state-sanctioned religion instead of relieving your biological urges with porn. So in the middle east, they want you thinking about Islam all day long. In the US, they want you thinking about Jesus n' Guns. In China, they want you thinking about the glory of Xi and the CCP. I'm pretty surprised actually that Russia hasn't cracked down on porn yet.

When you have lobby groups like the NRA, but no lobby group for the porn industry, this is what happens.