Bottom line, he has every right to be threatened. Your girlfriend is running around with a decent looking fellow who's a big sports star and she's regularly seeing him in his underwear. Now of course this is a soap so they amplified this a 1000x but yeah concerned or threatened is fine. Now how you choose to react to it can really go one of two ways:
1. Spoil the living crap outta of her and monopolize her free time as much as possible so prick boy doesn't get a chance
2. Spend your time, like Rex, trying to make him look bad and potentially have it blow up in your face.
Unfortunately Rex doesn't strike me as a number one option sort of a guy so he's really devoted himself to option number two.
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"If you were looking for something clever or witty or funny here, you've come to the wrong place."
I'm now upset with Adrianna, why can't she see he is trying to protect her. I hope Bo finds something on Tate soon, and yet I hope Rex moves on from Adrianna