As others have said, the problem is that Jill is making an unusually big request here (going on an unannounced date for reasons she refuses to disclose to the MC), yet she literally expects the MC to drop everything and say yes immediately. If he points out that he already has plans, Jill takes it as a personal affront - muttering to herself that the MC doesn't act like he cares about her when he tries to spend time with her later.
I agree she isn't a raging asshole as some have claimed, but she
is completely out of line to treat the MC this way. Realistically, it makes it clear that any relationship with her is likely doomed because it's only a matter of time until the MC is once again unable to dote on her hand and foot. I'm certain that's not what DPC was going for, but that's what he wound up writing.
In short, it's ridiculous for Jill to treat the MC more harshly when he agrees to tend to a sick friend than when he fucks that friend behind her boyfriend's back, yet here we are.
Here's the last time I tried to answer that question at length.
tl;dr The game has high production values, oodles of content, and tries hard to make us invested in the characters and their stories. Plus there's lots of sex, of course.
EDIT:
Technically, you don't have to be with Josy to be on the M/J path. You do have to go back for the date, but that's unavoidable and doesn't seem any more objectionable than some of the MC's early interactions with Sage or Isabella. But you can avoid buying Josy gifts or having sex with her in the early episodes and still get the the throuple (as long as you help Maya with her scavenger hunt).
It's possible that will have longer term implications, I suppose, but for now it works.