Frequently the villain will be the only homosexual in the story. For decades, this was the only role gays could play in mainstream media, and it still crops up uncomfortably often. On the other hand, for some people it is enough. Before adding an example, think of whether he'd be any different if he wasn't gay.
Or even better, outright threats of rape/molestation. His brand of evil must involve a sexual nature and/or some uncomfortable flirting. Please note that a villain simply being gay is not enough for this trope to be in effect. If he's not explicitly gay, but it wouldn't be surprising, he's a Sissy Villain. His motive is usually either his depraved sexuality or an unrequited love, like his Psycho Lesbian sister. Usually a villainous take on the Camp Gay or, on the other end of the spectrum, a Manly Gay sexual predator whose preferred 'quarry' are straight men or young, naive Twinks. The Spear Counterpart of the Psycho Lesbian, and the monosexual counterpart to the Depraved Bisexual.
