In the early 21st century, a man's choice of wearing an earring on either ear is a personal choice, regardless of sexuality. During the 1980s and early 1990s, a pierced right ear was associated with male homosexuality, while the left ear indicated heterosexuality.
By the mid- to late 1990s, as a wider choice of styles beyond studs became available, wearing a single earring on one side or the other was a commonly understood signal of sexuality. According to Jon-Jon Goulian in his memoir, "The Man in the Gray Flannel Skirt," men who wore earrings in their left ears were advertising not only their heterosexuality, but their liberal conscience.
Nowadays, it is common to find heterosexual men wearing earrings in both ears at once, as well as either their left or right alone. It is considered old-fashioned to wear a single earring as a sexual signal.