Jimmy is not an Alpha Male. He's a wishy washy cheater. I mean, he's almost cheating, but he's not actually cheating, because he's not actually doing anything that constitutes cheating, but he's making it clear that he is mobilizing to cheat if the (Super)girl feels like she wants to be an immoral person and do the dirty on Lucy, putting the weight of breaking Lucy's heart all on Kara's shoulders, which is a jerk move.
Cheaters don't stop cheating.
Kara could wind up spending the rest of their as yet unrealized relationship as almost Mrs Olsen waiting for James to cheat on her, because it in his nature to cheat and he can't help himself, so it's only a matter of time.
Tick, tock, cheat.
Win on the other hand was Ducky (John Cryer in the clip above) not enough bottle to put his nuts on the line by flopping out his feelings on the table and let the alien appraise some honest choices after being awarded devastatingly good intel, until yesterday. He used to be wussy, and now he's a real man. Forthright in the face of uncertainty about his intense ongoing romantic feelings for Kara.
Win grew a pair and told the girl to whack his love spuds with a mallet, if she wasn't interested in taking up what he was offering. The ball is in her court. So what if he's a soft nerd compared to Mr Football? Maid of Steel, Man of kleenex... Everyone on Earth is a soft nerd compared to a Kryptonian. What we got here is a bubbly little man who loves her completely and would never hurt Kara, and either kill or and die to protect her, compared to Jimmy who is a ragged piece of glass sawing back and forth across her honour from beginning to end. Winn is only not the full package because he's not handsome, he's got one of those squashed in a vice/pitbull faces, but he is sure as #### husband material, if he wasn't so ugly. And even then, his true and pure love for the girl from the desk nextdoor should make up for his assface, not that she's under any obligation to reciprocate or match all that love he's trying to give her, that she never asked for, or should any of that love emanating from him like stink emanating from old cheddar, make up for how dry his doughy shoulders leaves her nethers.
However, Kara needs dating instructions in large type.
Books on tape with the volume turned up to 11, even.
Win has never been rejected because he was never a candidate because Kara is a Moron.
He's fresh meat.
Jim on the other foot, is rancid and collecting flies from all the spoiling, dissolving his lack of resolve and mulched any potential that perfect smile hinted at in the beginning before he mentioned that he was taken by someone pleasant and brilliant and off the market to everyone who is not an asshole.
Supergirl's never tried to think about Win sexually before. Maybe when she tries, it'll turn out to be as neutral or as horrible as she hasn't even considered to think it might be about as yet... Or maybe she'll have a fantasy where Win rocks her alien world, and she'll want to move heaven and Earth to see that fantasy come true?
Lets say it one more time.
In the comics she dates and has sexual intercourse with a horse.