dear jerk:

I don't even know what to say. I'm legitimately speechless. I wouldn't have cared so much if you did that privately, but screaming it out to your whole house? That's beyond just being mad at him; that's hitting me, too. I cannot describe how embarrassed, how upset, and how hurt I am.
I liked you at first, but honestly, I think you're a pretty shitty person for doing that. This isn't between you and I, this doesn't even concern me, so leave me out of it. How was I supposed to know it was "yours"?
Get the fuck over it, grow the fuck up. You have to live with him for the better part of the next year. Man up and keep this between you two, not anyone else.
I haven't been this upset in a very, very long time.