You're most likely to hear them having sex on the weekend, but I think you might need to select the 'Set alarm for 7:00' option for the best chance? And once you do that, after waking you need to head straight for the showers.
I was able to encounter the GF in the shower (after I got up and immediately went to the stall), but since I lost that save IDK if the shower encounter will help progress the storyline.
There's also the scene when you go back to your dorm and find a sock over the knob, that will trigger a punishment for the roommate or for both, depending how you choose to reply..
