|Beyond the binary|
You should think that this was a meaningless questions, as there are enough bisexual men around to tell you that yes, they do exist.
That does not stop scientists from developing theories that deny their existence, though, especially if the fundamental thinking in a specific discipline is based on a different "truth".
The binary of sexual orientation
As far as I can see, the dominant ideology among many sociobiologists and biologically oriented psychologist has been that men are strictly homosexual or heterosexual, while women may have a more fluid sexual orientation.
This is, I believe, a result of the old evolutionary theory that men are constantly trying to spread their seed to as many as possible, while women are more oriented towards love and nurturing. This is why men are believed to be focussing solely on the visual clues of an attractive body, while women might even find footage of bonobos frolicking exciting.
This was at least the conclusions made by J. Michael Bailey and his team over at Northwestern University.
The return of the bisexual man
The International Herald Tribune now reports on a new Northwestern study, led by Allen Rosenthal (an associate of Bailey's, I believe), that indicates that bisexual men do in fact exist.
There are other studies pointing in the same direction: