From the game's standpoint, I think it's for exactly the reason that I mentioned; to give players on the non-M&J path alternate scenes with other characters at the same time as a M&J scene so the episode has the same level of content for both sides.
From a story standpoint, however, it doesn't seem to make much sense. I don't see any reason why the MC wouldn't still want to have a little fun with Cathy, especially if he flirted with he on Swyper. He might feel a bit sorry for her, sure, but why would that stop him from wanting to go a little further? It's not like he didn't just have sex 3-4 different women earlier, one of which he assumes he's in relationship with and the other who he knows technically still has a boyfriend, so this is an odd time to suddenly have a moral conscience. (Funny thing is that the scrappy mod appears to remove that barrier because you can get the Cathy scene even if you're on the M&J path, so it wouldn't be hard for DPC to also change it)
There's still a chance for Jade, however, but we'll have to wait and see if that is now completely closed off as well and I presume we'll find out in ep 6 either after a gender studies class, or if the MC is invited along to the councillor meeting with Rusty. However, if it is completely closed off, well then that's also kind of ridiculous because the MC could easily straighten things out with her, so I'm don't see how or why it could potentially end when there are still ample opportunities to try again.