Second, as was pointed out many times before, there seems to be something affecting the people at this college.
Yeah, the gate, which increase the lust (or perhaps just sexual freedom) of anyone around. And the longer you live near to it, the stronger is this increase. And so ?
Even if it's still not explicitly stated, it's not a secret and it's more or less well represented. That's why the teacher (who's near to the gate since so many years) don't need much to give up and act as a total slut. While in the same time the girl at the car shop is just a liberated woman ; she's way younger and she live further from the gate, so she's less influenced. And that's even why Eve is the boldest of the girls.
If you are going to fault one character for being unrealistic but ignore the multitude of other story flaws, [...]
The only real other flaw is Abigail in the last update. Yet it can be because her loneliness is deeper than I thought at first. So, giving up in friendship for the sole character who don't show too much hate for her make sense. Now that she feel betrayed even by her own sister, whatever if she thought that he is a total looser. He also is the only thing that prevent her to feel completely empty because of her loneliness. But the problem isn't that.
Each character is influenced according to the time past living near the gate, this balanced by its initial personality. Chloe is the typical, "I'll wait to encounter the love of my life before having sex" girl. The gate made her stop questioning her feeling ; she's in love, it's now a good enough reason. Anna is a girl who have nothing against sex, but still is faithful with her friends. The gate make her just put this faithfulness between parenthesis ; she don't care to fuck her friend's lover, as long as it will not hurt the feeling of Chloe. And so on for each characters... except Rose.
Rose is the typical nerd. That the gate made her lusting to Jason, alright. That, years after years, it made her give up to her lust and try to hit on him, it's also alright. But no, she don't go from, "It... it's a... a penis ?" to "yes, you two, fuck me !" like that. Simply, no ! Either she's deeply under the influence of the gate, and she should have gave up way before, or she just say "no, not you two at the same time !"
But the fact is that here her reaction is forced, to serve, both, as a trigger to make Anna more bold, and as a double script shortcut ; "in case you didn't noticed it yet, there's something strange with girls' sexuality here", and, "oh, have the power of the gate had a important increase recently ?". And, yes, it's a flaw, and a big one. Using Rose to express this is laziness and break the personality he tried to build for her.
Even the fact that there's no real need to search new girls for the ritual each year is not a flaw. The more a girl is sexually liberated, the easier she'll give up to the gate power and feel attracted by it. And these girls are the better candidates for (what ? We will know in the future), even if finally really few fill the expected profile.
I was stating that in this alternate universe, Rose showed an interest in Jason, as did many of the girls in the hallway, and the fact she knocked boots with Jason when the opportunity presented itself fits this story's narrative, if not your sense of how the world ought to work.
And I just explained you why it don't at all...