Spoilers for Aiden and Dallan's routes:
For NSFW Aiden 4-8, along Aiden's route, you should get to a part where you go to the headmaster's office to try and get hints about the test. Once you ask your questions, you can either choose to embrace it or choose to remain loyal to Aiden. There's also a separate set of scenes if you just skip to sex without asking any questions and ask to start with something simple. Aiden's route will be getting a rewrite after Scribbles and Alvy get their routes done, so this may change.
All of Dallan's NSFW scenes are of other people.
1: After leaving the shard meeting room with Dallan at the start, in the bathroom, choose a stall, and then prank them.
2-10: You need to accept working with Chelsie in her lab at the start of Dallan's route, then way, way, way down the line, you'll be in her workshop again and have the option to agree with what she's doing. A bit later, you'll be able to have sex, and there are a bunch of variations. Each of these options has a set of scenes:
No way => Chelsie alone
Yes: He's perfect => humanoid scene
There's something off about him... => furry scene
I'm not 100% sure about panels 11 and 12, but I think you have to go to detention with the group and then have sex with Chelsie, and regardless of which choices you pick within the scene, you'll get them. (You won't get them if you call her a pervert in the earlier scene or decline sex altogether)
One huge note regarding the entire game:
No matter the routes, I suggest you do not have sex with anyone outside of just wanting to unlock the scenes because each of the four main characters (Aiden, Tate, Dallan, and Alvy) like you much better when you're a virgin. The only exception is with Chelsie, which has no consequences because Dallan doesn't think sex outside of a relationship means anything.