Bisexuality is ironically stigmatized not just amongst some straight people but in LGBT circle. It's common for people to believe "bi woman are really just straight and bi men are just gay".
But when it comes to males and being attracted to both genders, many people doubt it. I recently thought about this as my workmate has a boyfriend who she was discussing is bi and she was a bit wary of that but accepted it. Most other girls said that he's deluding himself and it's only a matter of time before he starts shagging other guys/looking for them on Grindr.
I think that the reason why there are less bisexual men anecdotally and by statistics isn't because it's less common but rather the stigma.
In actual fact, I would have thought that bisexuality would be more common in males, the reason being is that men have a higher sex drive and are overrepresented in paraphilias and fetishes compared to women. So wouldn't men be more likely to have sex with their gender than women.