A person who changes sexual preferences and sexual identities alot. Someone who will become attracted to same sex gender at the flip of a switch, or "tossing of the coin". Someone who will randomly change their entire sexual identity without much thought (ie Hetero to Drag queen).
"Oh no! Donna, why is my boyfriend Dave sucking Bob's dick?" "Amy, didn't you know he was a coin-tosser when you met him? Yesterday he was dressed in drag and kissing my mother."