I mean I don’t know how many potential Democratic voters would prefer if the candidate wasn’t a woman, but if the last decade or so is any indication the number that would find it strongly objectionable is indistinguishable from zero. There’s no evidence at all that this is a significant concern; everybody seems willing to elect women when it comes down to it, it doesn’t show up in polls and the two examples that come up in this contexts both had much bigger problems than their genders.