First, follow the steps in the guide you can download on here to unlock CG #13 (the one with them laying down on the couch and making out).
The next morning, you'll be asked to define your relationship goals with Isaiah, and it needs to be either Sexual or Romantic & Sexual. If you tell Marco about Isaiah, it'll be one of these choices:
"I absolutely want to hook up with him." or "I want to hook up, but I think there might be more happening..."
If you don't talk to Marco, choose "No way we're stopping now!" then one of these:
"He'll make an incredible fuck buddy." or "I think there's more here..."
After that, it should unlock right before you take a shower.