Upvote:5
It's not usually illegal, but you should still be considerate of those around you. Some may not want to see nudity/violence/etc on a plane. Some of the inflight systems have some of the worst scenes removed(!).
However, I've seen people watching full on p**nography in the aisle seat, fully visible to others. Some just have no sense.
One point to consider is the laws of the country your airline is in, especially if flying domestically. If say the movie is banned in that country, you may want to avoid watching, and similarly with p**nographic movies or just movies with nudity.