It's one of those "unmarked" quest lines that's hard to find and complicated to get right unless you check the source code. In the end, it makes your roommate and roommate partner freely interactable/fuckable.
First, after 90 days have passed and you have seen most of the events with them, and you have no pending dare, and you have not canceled dares, you will see your roommate masturbate when you randomly enter your room; this only happens Monday through Thursday, mostly at 6pm, going in later will probably interfere with other activities the roommate is doing.
Then, if the masturbation event was relatively recent, during Friday-Sunday, their partner will come in (same requirements) to ask if they didn't break their dare, and you tell the partner that the roommate did, so you get a choice of either being serviced by the roommate (for dom route) or servicing the partner (for sub route).
Then finally two weeks later after you caught the roommate masturbating, the partner will be back in your dorm to make it official, depending on which route you took.