I've come to think of Quinn's jealousy reactions as a kind of spectrum, differing according to the context, type of girl involved, and the extent to which she's fallen for MC.
If we look at the hangout in episode 7, Quinn asks MC how many girls he's been with since coming to college; she insists she doesn't mean anything by it and that she's just "curious" (she'll use the same word again in the conversation outside the HOT house in episode 8). Then, when she doesn't get all the information, she says "at least you're using my services". So the context here is the restaurant, over which Quinn has a measure of control - indeed she's encouraged MC to use her services and offers him another freebie for good measure. It isn't that she's not jealous of Sarah, Melanie etc in this moment (asking the question shows she is), it's that she's rationalised it within the context of her business, and also the restaurant girls are more her kind of girl to begin with.
The Jill reaction is different and I like the explanation from
shazba above about what's going on here. The dumping scene can come across as a bit quick and casual but I think there's a lot going on, and in a sense this is backed up by what follows. By that point in the library I do think the MC has started to get past Quinn's defences and she likes him, but she's still really unsure of herself. When she overhears the conversation with Jill, she feels jealous, and it's worse because of the type of girl Jill is and the fact she's not on the restaurant menu. So to stop herself from getting hurt she dumps him. This MC has also given her the perfect excuse for doing so: here's someone she thought she was into but actually he's not who she thought he was, because he's trying to date goody two shoes Jill and go to fancy dinner parties. The consequences of this dumping are telling - whilst an MC who never helped Quinn in episode 6 can still get the threesome with Sage and Quinn even though Quinn "doesn't even like" him, a dumped MC cannot, even if he's done all of her prior scenes. A woman scorned. Brutal.
Then there's Sage. We've already seen Quinn act jealous when she catches MC and Sage in episode 7 so we know she's bothered by it. Why doesn't she dump him? I think it comes down to who Sage is and timing. Sage isn't a restaurant girl, but she's also not Jill. Quinn can understand an MC who is also interested in Sage, and it doesn't contradict what she thought she knew about him in the way the conversation with Jill does. But the key reason is what happens on the bench outside the library...
Nail on head. The significance of the first bench scene can't be overstated, the second big hinge point on Quinn's path after helping her in episode 6, and I think it marks a key moment when Quinn begins to admit to herself that she's really into MC. So by the time of the second bench scene, after party planning, when Quinn asks MC about Sage, (again because she's "curious"), things have gone too far for her to just dump MC out of jealousy. It might have been what she'd have done before, but as it is all she can do now is lean into his shoulder.
