This - be decent, and even more importantly - call out "indecentness" when you see it.

Call it out to 'friends', to family, online, in person - even if all you feel you can say is 'that isn't right' - say it - you don't need to get into fights, you don't need to be a saviour, you just need to call it out. You can use that as excuse to leave, or you can use that as a reason to stay.

But don't just ignore it.