The emerging Gay community, really coming out of its societal closet for the first time in Western culture was still largely in danger every day. Being overt was tantamount to walking up to someone and asking to be beaten, or thrown out of one's home. So a whole dialect of the Attire language was slowly born that aided both in gay men identifying each other more easily, and with greater confidence; and in some cases within that dialect, simplified the process of determining whether another man was a suitable potential sexual playmate.
The visual tropes that developed were so successful, that they have become iconic images of what people perceive that gay looks like. Though, naturally, the reality is quite different, especially these days.
Here are 6 different images from Fischer's work Gay Semiotics.
Basic Street Fashion, soon to be called the Clone look, so named because it became without question the ubiquitous look for gay men all over the USA; rapidly spreading to other nations. This look, unlike others in the dialect, continues to be a go to for many gay men. And its interesting to note that it spread to the heterosexual community, and is now part and parcel of the male hipster wardrobe right now.
Jock fashion was a strong look within the Gay Community. Whether the gent in question actually played sports, or was only trying to look like he did was immaterial. What identifies this as a gay signal set is mostly the tightness of the clothing. That, and the average straight guy wasn't going to be running around the streets in work out gear. But the super short, super tight satiny shorts were a commonplace sight within the gay community. The whole look communicated a youthful energy and a connection the masculine pursuits of sport, though filtered through the unique lens of that growing sub-culture.
In addition to what you're seeing here, there was the universal hanky code. The hanky code used the many colors of cotton bandana handkerchiefs to operate as a sartorial semaphore system. A man using a particular color in a particular pocket of his jeans or leather pants was deliberately advertizing certain sexual interests. Left pocket, dominant, right pocket, passive. Light blue hanky, oral sex, navy blue, anal sex, red, fisting, and so on. The list is long, and sometimes a trifle confusing. Suffice it to say, if you've never looked at the hanky code listing, you're bound to find a kind of sexual interest you didn't know about before.