I think you need to start with the impossible, define porn/nsfw.
Why is lingerie provacative and bikinis aren't? Why is nudity even a problem? Why nipples and not elbows... Ultimately, this stuff is the very definition of subjective... something that is entirely determined by perception. What is and isn't "safe for work" is constantly changing. Taboos allow dishonesty to continue.
Call it homophobia if you like, but I call the whole thing conservatism.
Personally, I think our rapidly changed attitudes about homophobia and transgenderism is closely related to sexual liberalism generally. Without online porn, gay culture could easily have stayed closeted.