Perhaps it's because of posts like "Photos" on this very same forum.
I always thought it was because of the service innuendo. You know your own personal slave that's gorgeous and in uniform. I agree with flyblue it's how you portray yourself as to whether the stigma sticks. And over the years I have met other crew just like that but certainly not the majority.
Same garbage as "all male crew are gay ". It's usually carried on by people who aren't in the industry or think they know people who are. I think they all need to get a life.