You signed up for this, so going green during the act is not a preferred option. If you were the kind of kid who gave your friends a piece of your cake when they asked you for a bite, and then immediately asked for it back, consider whether or not that possessive streak might have stuck. You say you're cool with seeing her with someone else, but are you really? 

There's nothing wrong with not wanting to sharejust own up to it from the start. There are plenty of other ways to spice your life up that don't involve another person.