I have heard it said that it began with “Three’s Company” which first aired in 1976. The John Ritter character, although not gay in fact, pretended he was so as to be able to share the apartment with the two women. I rarely watched the show, so I couldn’t tell you too much about it. This was 36 years ago!
I’d say it started with Liberace and Paul Lynde. Innocuous enough back then, but everybody knew what they were.
And how about Tom Hanks in Bosom Buddies?
Yep. It's been slow conditioning. Like a frog being slowly boiled alive. I got out of the water in 1997 and have discovered it was already pretty darned hot by then. Many frogs have already been completely boiled and they don't even know it. Try arguing any subject rationally with them and you will experience what I mean, first hand.
Don't forget Billy Crystal in "SOAP"....