People often assume people they meet are heterosexual, in part because entertainment media (i.e., television, movies, music) often fail to represent homosexual couples in a positive light, if at all.
Society favors masculinity. Men are paid more and respected more. They are represented more in media and professional environments.
White people are represented the most in media, so they have the privilege of having positive role models in every direction. It is easier for a White person to shop without being suspected of theft, to find somewhere to get a haircut.