Here's an odd thought that just floated through my head: Winn isn't even straight, or if he is, he isn't necessarily interested in Kara that way. He's jealous because he was friends with Kara at their job long before she revealed her powers, and now he sees some of what is going on - and particularly Jimmy - as encroaching on their friendship. Which might explain why his rage monster took the form that it did: he didn't say he was upset about possibly losing her to Jimmy, his core issue is that she's never going to be normal. He wasn't trying to rub that in her face - that was accidental - he was expressing his upset that because of who and what she is and is becoming, he is losing his friend. Possibly his only one - we haven't seen any indication that he has any social life outside of her, and he doesn't have family ties, really, either.
I'm probably completely wrong. But time will tell. Or maybe I've already missed something.
I'm probably completely wrong. But time will tell. Or maybe I've already missed something.