Affinity is tied to "major choices" and isn't affected by all those mostly conversational options you get regularly, that's how. My best guess is that you've made those decisions thinking they'll count towards the same scale and you could course-correct by just giving a bunch of CHICK answers (which you did since you were massive CHICK).
In order, the CHICK outcome of these major choices up to the M&J scene are: not beating Troy, not accepting Quinn's "fast food" offer, talking to Steve in the store before the date with Josy, running away from the jocks after successfully beating Dawe, letting Derek do the DIK stuff in the Pink Rose club, stopping Jade when she makes a move during class, and lastly, refusing to smoke weed with Quinn on the rooftop.
You don't have to pick all of these since a neutral affinity is enough to get the "threeway" option. Doing maybe 4 CHICK and 3 DIK is pretty safe so you still get to have fun, just not all of it.
EDIT: You probably don't have to replay the whole game again if you've kept saves from earlier episodes. Just load up a save file where you still have a few of these major choices left to make and go with the CHICK option.