I'm afraid my relationship is unraveling...?

I believe your girl friend is not really a keeper. She is interested in your relationship and probably loves you, but she is not serous about it and she still want to have fun and get other guys attention which might lead her to accidental cheating, no one has the right to sexually joke around with other when they are in a relationship with some one specially to the point that she went. If I were you I would just leave her and find some one who actually and truly cares about me. you sound like a sweet guy and you deserve better. and BTW i think you're under reacting.