As far as I like feminism ( I wrote a fun thriller about an overweight policewoman who sleep with whoever she like, and doesn't care... actually top100 in french amazon)
As far as I consider women should have sex as they please without being judged, I think the study is bullshit.
Women fucking around are labeled slut cause they take some sort of physical risk. Sleeping with an unknown man could be dangerous. While sleeping with an unknown women is probably less risky for a man.
So anyone willing to take even a slight risk for sexual pleasure is logically labeled a Slut. It's more or less a stupid move.
A correct procedure in the testing would, imho, involve lesbian and gay:
Are lesbian having casual sex (less risky than heterosexual sex) still labeled 'slut' ?
Are gay men having casual sex (more risky , with my premise, than straight) still NOT labeled 'slut'?
With such a testing we could see if the stigmata of girl fucking around is really about gender education and machismo, or , as I suppose, about judging people for the amount of risk they're willing to take for some sexual pleasure.
As it is, the study tell us some evidence: girl are more badly judged than boy. What the study doesn't tell, and should, is why.