Why won’t the LPGA let a golfer play on the grounds s/he was not “born female”?

I thought gender was just a social construct? What difference could it possibly make that this athlete was born male? And why is this Old Girls’ Club so sexist?

“Back in 2008, Lana Lawless crushed the competition in a women’s long-drive golf world championship – smacking a drive 254 yards into a headwind.
She may have had some help from her chromosomes.
Until 2005, Lawless was a man – a retired police officer who went in for transgender surgery.
After her big win, Lawless was banned from Long Drivers of America. LDA adopted the LPGA’s rule that competitors must be born female.
Lawless is suing.
“It’s an issue of access and opportunity,” Lawless told the New York Times. “I’ve been shut out because of prejudice.””

http://www.businessinsider.com/man-turned-woman-suing-because-womens-golf-league-banned-her-after-she-crushed-the-competition-2010-10