How did this notion originate, and is it still current?
Never heard of it.
I used to wear my keys on a carabiner attached to a belt loop because I didn’t like them poking me in the thigh when they were in my pocket. No one ever assumed I was gay (I’m not) because of it.
LOL
This subject is such a crock! I remember back around 1975 when it became an issue. Some lady wrote a letter to a magazine–The Atlantic or something lke that–complaining that her husband had to carry a big ring of keys at work; because they tore holes in his pockets, he took to wearing the keys on his belt. That caused him to get hit on by gays when he went to bars. Until he learned of the “code”–left side keys gays, right side straight. So he switched his keys to the right side. Fine, until they changed the “code”–now left side meant “submissive” gay, and right side meant “dominant” gay. Still getting hit on by gays! So it’s back to tearing holes in the pockets!
Then it may be a notion that’s passe. I think it was more of an 1980’s thing, but at that time was so prevalent that once, when David Letterman was exhibiting some possible holiday gifts that year, one item was a belt-loop keychain which prominently displayed the legend “I’m not that way”.
Speaking for myself I’m always reluctant to make assumptions
about people based on such stupid things, but there was definitely a widespread popular notion regarding this at the time.
I never heard of it either.
Keys worn on belt generally mean that you have too many keys to fit in your pocket, or you use them a lot and so wear them where you can get to them easily [like many maintenance men].
Now maybe, if you wear pants so tight you CAN’T put you keys in the pocket and have to wear them on your belt, then you might be gay.
Also, if you have to carry a lot of keys in your pocket, it’s going to be uncomfortable no matter what kind of pants
you wear. I don’t have to carry a lot myself, but I generally put them in my briefcase to avoid losing them.
Does this mean that every single janitor I have ever seen is gay? 
That’s what I was thinking, Xizor-it’s always custodial staff, and wardens and the like who wear keys on their belts…
Weird.
You guys are missing out on an entire subculture of the gay community; fortunately, it’s all passe.
Yes, at one time, wearing keys on your belt was an indication that you were gay (it was a limited, regional thing, IIRC). And yes, depending on which side you wore it, it also denoted whether you were top or bottom (aggressive or passive).
It’s all related to the hanky codes (one such list is here, although different lists have slight variations - even on this one, keys meant something specific, not just gay/top or gay/bottom) - depending on the color of the hanky and in which pocket you wear it meant different things. Check gayscape.com or some such queer search engine for the history if you’re really interested. Suffice it to say it’s rather out of date in this day and age of sexual openness and celebration, although some in the leather/bondage/S&M communities still use them.
One not listed, my personal favorite: Holstein patterned handkerchief, left pocket = looking for Gateway 2000, right pocket = has Gateway 2000. 
Esprix
Is that for real?
Damn, I knew being gay wasn’t easy, but you guys must have to carry cheat sheets!
That can’t be for real.
Imagine the embarassment of having your light blue hanky confused for a robin’s egg blue hanky. Aren’t they basically the same colour?
So, are you trying to tell me Schneider the super from “One Day at a Time” was GAY? No way!
Well… don’t tell anyone, but you’re not allowed to be a full member of the tribe until you can name the hanky codes, the meaning of every color of the rainbow, and the menu for a perfect brunch.
<giggle>
Lesbians, of course, can substitute any of those requirements with owning every Holly Near or Cris Williamson recording ever made.
xizor - Yes, all janitors are gay as well as a high number of prison guards and their supervisors. I thought everyone knew this.
Esprix - I just took my keys off the right side of my belt, there’s 12 of them on a short (removable) chain connected to a climbing clip. I usually carry my multi-tool on the other side.
What would this have made me back in the seventies?
I just realized that the chain by itself might have some decent uses… heh heh heh
(I’m really just an obsessively organized straight guy and yes I use every one of them there keys and the multi-tool on a near daily basis).
I’m so lazy about remembering things like that that i’ll never get to be a full member of the tribe!
Dr_Paprika & Sue Duhnym, the codes were quite real. Remember, in the 70’s, the catch phrase was “sexual liberation,” not only in mainstream culture, but especially among the newly-juiced gay community (Stonewall was 1969). Still lacking focus, but stridently avoiding “emulating the heterosexual ideal,” sex was it, and somewhere along the line (I don’t have the exact development handy) the hanky codes came into being.
As far as differentiating colors, somehow or another you could tell them apart (but usually when they were side by side). And you didn’t have to memorize the entire list, only the colors that interested you. Made picking up quite easy. Keep in mind, too, that a lot of similar colors were related activities. Plus gay men are experts at telling the difference between beige, egg white, off-white, pale white, white, tan, and light brown.
Feynn, according to the chart, keys on the right in the front meant you have a car (i.e., you need to go to someone else’s place to have sex). The multi-tool… well, I think we can all figure out what that means…
And Doob, if you don’t know, let me teach you…
Esprix
I came from the suburbs, where blue-collar types wore keys on their belts (b/c they had custodial/security/etc. jobs requireing them to carry lots of keys). Then, in the ‘90s, all the young hipsters started emulating the blue-collar fashion: gas station attendants’ uniforms, etc., including the big ring of keys on a chain. I think this flood of middle-class silliness probably drowned any lingering connotation that keys on the belt=gay. Now it just means “hipster doofus.”
Just as, at one time, a guy having an earring meant they were gay, then an earring in their right ear made them gay, and now it just means, well, not very much.
As usual, the straight community usurps gay fashion - a few years late. 
Esprix