On UA they state that you need to use the Lav in your cabin because we can't allow people to collect around the cabin door. I am not sure if CO specifically states the "why" part.

All in all I understand the reasoning but if there is no one in the F lav and no one in line I really don't see how a pax in the first row of Y gets up and uses the F lav can be a security threat. Yes there is the mentality that if you let one person do it you have to let them all, but I have seen an FA tell a person to use the Y Lav maybe 1 out of every 4 times I see it. There is also the mentality to maintain the F allure so to speak, but seriously if I'm in F and one person from Y uses the F lav I'm not throwing a fit, if it were 10 then I may take issue.