I just scanned my email address book for fulltime liveaboards and found that about 2/3 are couples, the remaining 1/3 are singlehanders split about evenly between women and men. So, my totally unscientific sample says 15% are women singlehanders on big enough boats to live aboard.
Their stories are as varied as they are: one is a snowbird who moves from Annapolis to Florida with the seasons; one started the Great Loop solo, fell in love with Georgian Bay and now keeps her boat there (bought a house in Florida for winters); one has two boats(!) a dock queen near her job in downtown DC where she stays during the week, and a sailboat here in Annapolis where she plays on weekends - her idea of vacation is to fly to some exotic location like Australia or the Med and charter a bareboat; one has lived aboard many years and done some cruising, she's getting ready to head to the Carib when she retires next year; two are dock queens looking for cheap rent though they talk of cruising someday I don't see them developing the skills to make it happen; one just moved ashore due to cancer. I'm in another online group of women sailors that boasts over 250 members. So perhaps not quite as small a group as you were led to believe!
Of the couples, I know of two for sure where the woman was the primary impetus for getting the boat and is the more knowledgeable of the two. But that doesn't mean that the woman is the lead in *only* two, most of the other couples, I simply don't know the dynamic. Again, anecdotal data and likely skewed as we tend to associate with couples that are fairly egalitarian; if one partner were totally subservient to the other they probably wouldn't be in my address book.