It's only a problem if they are not aware you are watching
Leaving the door open is one thing, if you're walking down a hallway and they obviously aren't trying to hide anything, I believe it cold definitely be labeled healthy,normal arousal. If you are watching from some tiny hidden video than no, it isn't healthy at all unless they are aware and desire or at least willing to not care whether you watch.
Otherwise it is violating your other people's privacy, and out of all the private activities for you to violate -- sex if probably the worst one to be violated -- but that's my system /belief on severity of privacy violation, the only thing worse would be to then further violate by sharing or inviting others.
But honestly that's a moot point because that is not what you are asking, and even if you were, it would be their call as to whether or not they are offended.
Just make sure you don't interupt, and ask before you approach