Okay, if that's the easy to come by data we're going with, than all those guys who identify as straight but engage/have engaged in MM activities are excluded from the figure.
But you claim these guys are not really straight. Thus that US census data, according to you, doesn't answer the question "How many men are actually bisexual?" at all.
Whoops.
The very nature of the question means that survey data can't be used. I think we can all agree, however that whatever the self-reported data for men is, the real number is higher. IMO the real number for women is actually lower than self-reported due to the endless number of girls who go out of the way to say they're "bisexual" but never seem to be dating/hooking up with any women...
Shockingly, people respond to incentives. Identifying as a bisexual man has no/very niche upside in terms of real world incentives. It's the opposite for women.