As Ezra says, I think it's clear that a lot of the Jim Webb fanboys in left-of-center ranks are significantly driven by a somewhat Chris Matthews-esque infatuation with his Acqua Velva scent. On the other hand, a lot of what's driving pro-Webb sentiment is a meta-level sense that these qualities will appeal to other voters in much the same way that various liberals thought John Kerry's war record would make him appealing to other people.
And, of course, as I've said before it's hard to even tease these things apart, because people have a tendency to project onto swing voters a desire for the exact same qualities that they themselves desire. One interesting twist on this, however, is that the most prominent pushers of a strongly masculinist conception of the presidency haven't been Obamaphile Webb fans, it's been a certain segment of Hillary Clinton supporters who in a weird way seem to have decided that backing a woman candidate give them carte blanche to be as sexist as they wanna be in arguing that Barack Obama's too effiminate to be president.
Last, it's worth noting that the best evidence available suggests that women who acquire a major party nomination don't face any unique disadvantages. Instead, we have few women in elected office because they don't run as often largely because of structural barriers. I think the apparently widespread assumption (one contradicted by the fact that women who do run seem to do fine and, of course, the fact that most voters are women) that the public is craving hyper-masculinity counts as one such barrier and it's one that's massively restricting the talent pool for progressive politics since a large majority of liberals are women.