Bisexuality is less common in men than in women. Women have what is called a [i]fluid sexuality[/i], meaning they can easily go both ways. On average, a man's sexuality is more rigid, making it harder for a gay man to go straight and vice versa.
I was trying to add on that. Psychology states that it's harder for males to go both ways, which is another way of saying they're bisexual. It's a possibility, but you're right, it's rare.