A good orgasm if mental but a great orgasm, the kind that makes your legs week and makes you glad you're laying in your bed cause you know you can't move much less get up, is physical.
I think bratcat said it best. Your mind gets you hot, and your body's natural reflex is to release.
I have always wondered exactly what orgasms are. I know they feel great, and make me feel great. But who knows whether it's mental, physical, or a combination. But my guess would be 90% mental, 10% physical.