I'm done with this post as soon as I type this reply.

No matter what any of us say, you will defend this boy and his actions.  Clearly, the only one doing anything wrong is his girlfriend by merely existing and for falling for him in the first place, right?

Give him the ultimatum and you will be shocked as to how fast he disappears from your life.  You are only fun right now because you are "off limits" and "dangerous" by being the other girl in the equation.  Your "romantic' man is a master manipulator because the only one that will be upset by the end of the affair is YOU.

Finally, girls like you make my head hurt all the while I'm mourning the future of my gender. 

Good luck with all this and don't say we didn't warn you.