Its good your position has moderated from the OP.
Ive never seen anyone watching porn on the transit. Sounds to me like its either a form of sexual harassment by exposing unwilling participants to sexual activity or people are too poor to afford a private space to do private things. If the former, then its more like flashing and the fact of the phone being involved is incidental and not the same as music at all. If the latter, then idk it sounds like moral panics about people pooping in public or gay park sex that are really displacing discomfort about the oppression of other people onto the people at the bottom of some sort of hierarchy. People have been watching porn at libraries for a long time. And we used to have things like porno theaters or bookstores where semi public jack off space was provided, and only exposing those who are consenting, but those do not exist as much anymore. Rent is expensive and people are in tight spaces.
Someone enjoying music (even music you dont care for) is totally different than sexual harassment. Just because they share an item in common (phone) doesn't mean its fair to make an equivalence. Its diminishing of sexual harassment to put it on the same level as music.
If people are just being inconsiderate then give them some kind of feedback about it. If it is really true that you have approached people and they've all flipped out on you, its provably something about your approach. I do share unsolicited opinions with people in public and more likely they just ignore me. If I introduce hostility then people react in kind. You can just mutter passive aggressively to yourself or another person you dont even need to be direct, and the person might get the message and be different next time.