Aside from my silly response awhile ago, in all seriousness I think it tends to do with the appropriated stigma from times past.  Years ago (you know, the 20th century) men and women had distinct roles within the world.  Women were either the homemaker or they held jobs like nurse or teacher.  Men were the breadwinners and did the big "manly" jobs like steel mill worker and farmer.  (OK, so those were gross overgeneralizations but sometimes simplicity works best) I think it was once the lines between what were acceptable occupations for which gender that the stigma of designer = gay became the norm.  In all reality trying to prove that assumption is like proving that the DSquare brothers are not really twins or that CoCo Chanel didn't formulate one of the most remembered fragrences ever. 

I'm an interior designer by trade and yes, I enjoy the company of the same sex (hows that for a euphemism?) but in my day to day I've met many male designers who were married (and not just for green cards) as well as female contractors and construction workers. 

But on another note, I think that modern day media does offer its perceptions - think about it - the squeaky wheel gets the grease.  Which designers do you see on tv with their own shows, etc?  Its the gay ones, the amazingly flamboyant gay ones.  SO how are people NOT to make that assumption?  Like I said, isn't what is protrayed on tv the absolute?