A former heterosexual person, who only discovered their homosexuality because of a homosexual partner. They are referred to as a pancake because they were "flipped" to the other side.
Stacy was straight, until she met Jill. Stacy didn't understand her crush on Jill but went along with her feelings. They eventually started dating. Stacy is a pancake.
*Jill, in this example, is a Spatula