Usually, it referrs to two gay guys in the closet, but it's beginning to become a legitimate psychological term that holds a new meaning. Because the stigma of being gay is slowly evaporating, more and more straight men understand that you can be good friends with a gay man and leave it at that. Mostly, but not entirely exclusive, men who have platonic female friends are more likely to have platonic gay men as friends. The gay man vary well may be attracted to that straight man in the friendship, but attraction is not equal to love. A straight man can love a gay man without there being any sexuality involved. It may be rare, but it exists, and it's becoming more common. The only thing that holds this back from becoming more common is the fear of being viewed as gay even though you know you're not.
Therefore: only men who are truly comfortable with their sexuality can be legitimate bromosexuals.
Friend #1: I've never seen a straight man and a gay man as friends. They must be gay.
Friend #2: Nah, they just don't care about labels, and they're just bromosexuals.