One of the original Boomers, Stallone began his career in 1970, at age 23, in the hardcore porn Party at Kitty and Studs. Repackaged as The Italian Stallion during the 1990s, it became the bestselling porn dvd of all time.
Stallone was in a number of films during the early 1970s, including Lords of Flatbush, but he hit it big as the underdog boxer in Rocky (1976). Many starring roles followed, some straight policers (Nighthawks), some comedies (Rhinestone), one trucker-arm wrestler (Over the Top, with David Mendenhall), but Stallone was at his best as a man-mountain who grunts, snarls, flexes, and saves the day.
Movies by other man-mountains -- Schwarzeneggar, Lundgren, Norris, Van Damme -- drew the crowds with their reactionary political agenda, heterosexist girl-rescues, and climaxes where the effeminate gay-coded villain gets his just deserts, so there wasn't much for gay fans to watch except the beefcake. But Stallone was different.
Even his beefcake was different. Other man-mountains were rarely nude -- you don't need nudity to demonstrate that you are strong enough to lift a small country. But Stallone never shied away from frontal and rear nudity. His muscles were objects of utility but also of beauty, offered to both male and female fans.
Nor is Stallone homophobic in real life. He is so gay friendly that when his son Sage Stallone died tragically in the summer of 2012, the homophobic Westboro Baptist Church picketed his funeral, "accusing" the elder actor of teaching him tolerance.