It's obvious...
I had a Lufthansa flight attendant and his girlfriend as passengers in my Cessna 404 once. I asked him this obvious question, thinking that it wouldn't offend him since he was not gay, per se.
He replied that gay people were an obvious choice: they usually had no family ties (wife and kids) that made rostering a problem and they combined the physical strength of a man with the caring nature of a woman.
He went on to say that his company had been hit with a serious AIDS problem as many of the gay employees went down with this unforeseen disease. (This conversation took place around 1988; I assume that AIDS is no longer such a serious problem to gay people.)
I think there are a disproportionate number of gays working as flight attendants for the reasons stated. On the other hand, I don't see any problem with this possible fact. Every so often you do get someone 'camping it up' a bit on the PA system but that is about all.