Copied directly from my pharmacology for nursing care text:
"Yawngasm. Rarely, patients taking clomipramine [Anafranil] experience yawngasm. Experience what? A spontaneous orgasm while yawning. Honest. This unusual side effect, which affects both males and females, may be considered adverse or beneficial, depending on one's view of such things. In at least one documented case, yawngasms strongly influenced adherence, as evidenced by the patient asking how long she would be "allowed" to continue treatment. Although data are scarce, one might guess that the occasional yawngasm would help relieve depression."
I seem to remember reading something about an anti-yawning drug that had this same effect, but it's name eludes me.