I was on a redeye flight going cross-country when I realized that the man beside me was doing something questionable.
I panicked, and pretended I was asleep. The lights were dimmed, and everyone else appeared to be asleep, so the thought of me alerting an attendant and knowing it was an overfull flight etc etc made me decide not to react.
In hindsight, I know I should have said or done something. He finally left to “enjoy time” in the washroom, and I avoided any eye contact the remainder of the flight.
In researching this, it’s apparently more common than we realize, and people like me who do not report anything are the majority. There’s little the airline can and will do.