Wanted to share my advice, which is a contrarian view point to what others suggested:
You may not wanna hear this, because ur generation (unfortunately) was raised with the warp notion that "everyone is a winner" (even when u lost).
Thats not reality or how the world is
There is "totem poles" u will encounter; some in the locker room.
Your roommate is packing a bigger piece of meat. Thats just a fact.
I bet if you started showing him some respect (privately) and said things like "damn dude, ur hung like a fucking tripod, how do girls even take that beast & walk the next day?!"
I bet ya, horse hung roomie would appreciate that ego boost and your change of attitude!! I KNOW he would man.
And then guess what? he'd STOP embarrassing / bullying you, cause u would be now his bro / wingman with acknowledgment of where yall stand.
So "kill him with kindness"... trust me, it will work.