I hit lefty, but only in baseball 

A metaphorical way of a telling a gay or lesbian you are straight and have no desire or capacity for a same-sex hookup.
You tell a bi I switch hit, but only in baseball.

At the batting cage....
Lesbian standing outside cage: I just LOOOVEE the way you swing the bat left-handed. Can you hit lefty with me?
Me: Hell, no! I hit lefty, but only in baseball! I I throw righty.

I hit lefty, but only in baseball by Stop the Pendulum April 25, 2006