100-150 years ago these were also exclusively male names: Shirley, Lindsay, Gale, Leslie, etc.
I hate MadiSON for a girl, ick!!!
Okay, I'll tell y'all how this came about.
As some of you Southern folks may know, it is a very old tradition among many Southerners, as well as in a few old Northern families, to give a child of either sex its mother's maiden name as a Christian name. For example, if John Smith marries Caroline Rivers, their first child is named Rivers Smith no matter what type of genitalia it has. So when you meet a guy name Armistead Lightfoot, if you know anything about history you know whom he's related to and can peg him right away. If you live in Virginia, where everybody is everybody else's cousin and most people are genealogy psychos, you may even be able to figure out how you're related just by his name. You can even strike up a conversation with him by saying, "I think your fourth great-grandfather served in the Seventh Virginia Cavalry, Company A, with my fourth great-grandfather's brother!" It's very convenient.
Now, folks who don't have kin amongst the Lightfoots and Armisteads think this all sounds very classy. They're not proud and content to just be themselves, the best selves they can be, so they try to give their kids more high-falutin' names and maybe make other people think the kid is from a snazzy background. They give their girls names like "Madison" and "Brett" and "Blaine" because they think that sounds upper-class.
P.S.: I think the use of "Madison" came from that TV show Cybill Shepherd was in in the 1980s. The writers gave her that name to make her sound aristocratic.