I admit that I have never understand why folks are bothered by coach passengers using the First Class lavatory, but clearly from reading here and having seatmates who complain about, it does bother people.
To my thinking, beyond potentially delaying another passenger from having access to the lavatory, it is not like they are eating first class meals or raiding the First Class beverage cart (which IMHO would be a bigger issue).
Maybe it is my disposition, but there are far worse things that happen on-board aircraft than coach passengers coming forward to relieve themselves, but again I do realize it bothers other passengers and for the most part, I fly airlines which do police use of the forward lavatory.
It also might have something to do with flying many flights from PBI and to a lessor extent FLL where many older passengers have enough trouble making it to the lavatory without having an accident in their pants. Furthermore, on at least two flights from PBI an older passenger came forward to relieve themself, were sent back to coach and then proceeded to have an accident in their pants and we had to smell it for the rest of the flight.
Again, I more than realize that it does bother other folks enough to warrant airlines installing ropes and curtains, but I myself am just not that bothered by it, nor Alaska’s weak enforcement of who can or cannot use the forward lav.