I know a girl named Sean. Her sister is named Sidney.
Quick Wiki: looks like it’s historically unisex, maybe making it one of only a very few names that have been used as unisex long-term?
Sure, but show me a girl named Otto. 
There are no stable unisex names. Once a name becomes unisex, it is doomed to become female over time. {though I’m sure there are always exceptions…I’m sure there is at least one boy named Sue out there 
}
I don’t know why it is. Parents of baby girls continually encroach on boy names…and when they succeed parents of boys will tend to avoid those names from then on.
No.
It only goes one way…
…because people think females, or anything “feminine”, is inferior. Thererfore, once a name becomes feminine/inferior, parents cannot give an “inferior” name to a male.
Not only names, but colors too. Pink, purple, most pastels, etc. are now feminine colors.
Riding horses used to be a “male” activity back up until the 1960’s. Today, horseback riding is now mostly a girls sport/activity.
(very similar to : … its ok/great for a girl to be/act/dress a tomboy, but its shameful for a boy to want to act/look feminine )
It may be subconscious, people may deny it, people might not want to admit it… but its there.
I know females named Sean, Rory and Toby. I also have a cousin named Madison.
I live in a place with a plethora of odd names, and I am not sure what would or would not make the list. I am pretty sure I know some odd form of Justine like Justyn or something, but that was a long time ago.
We know a girl named Aidan who is my son’s age(5).
We also know half a dozen boys with that name as well.
I have heard of girls named Sean for years.
Most were spelled Shawn, or Shaun, but some were spelled Sean.
Following the same logic then:
People think males, or anything “masculine” is superior. Therefore a masculine name is superior, and parents of a girl will select unisex or somewhat masculine names for their child in an attempt to elevate her status.
No idea if it’s the true reason or not, but following from Susanann’s assertion…
Pink I’ll give you (formerly masculine as a shade of red), but Purple was claimed / taken / used by the suffragette movement, and may well have feminine associations because of that.
To the OP: Jody doesn’t surprise me was a name that’s gone feminine, and neither does Stacey. Toby seems a little odd (to me), but given the similarity of form – the “y” ending – it might have only been a matter of time.
Mary, Sally, Judy, Kathy, Jody, Stacey, Toby, Harry…
Dr. Elliot Reed
I knew a female Brian once (though it was spelled Briann, and not pronounced Bree-anne).
I had a customer bring in her baby the other day. The baby was named “Bradley” so I thought it was a boy. It’s not. It’s a girl named Bradley… and I’m an embarrassed bank teller.
Is this just an American thing?
As a Brit so many of these just sound utterly wrong to me.