Here bees my thoughts on this:
* Not all women are scared of bikes; I dated one who loved them, but she was unusual! Some women are great on a bike - as the rider or the passenger. But I think they are in the minority.
* Men are (usually) much stronger, physically, than women - and riding a bike (especially a sport bike at a fast pace - or a big cruiser, period) takes physical strength. On a sport bike, you have to have reasonably strong arms and legs to lean the bike, move it around, etc. And a big cruiser just requires big arms (and legs) period. A 140 pound woman and an 800 pound cruiser do not mix well.
Also, I am willing to bet that men - in general - have a superior sense of spatial relationships and are more eye-hand coordinated than most women - another factor that is very important as far as being comfortable and confident (and so, less scared) on a bike.
Finally - the Big One:
* Most women are, by nature, more risk-averse than most men. Where most men look at a bike and think about fun (and challenge) most women see vulnerable body parts and horrible injuries. It's why men make better explorers and leaders (again, usually) while women are better with small kids and talking care of others.
It ain't politically correct, but it bees de troof!