You can't progress with The Best Friend if you're already dating someone. Even D/S relationships count. This could be it.
Also, in every save I've played, The Best Friend is usually Open. Maybe you must have the Open Relationship inclination as well?
Have you solved the mystery at the quad party/been helped by The Best Friend in the dorm? I'm guessing yes since it's virtually impossible to have sex with them until you do one of these events... My main guess is you're already dating someone or you don't have the Open inclination.
I don't know what you're talking about. Because I always get into a relationship before dating my BF. You just have to complete their story quest. Which means you've gotta A) be attractive to them so you can get the shower scene. Or B) Solve the Mystery of the Party Milkshakes. And then spend time in the afternoon with them talking about sex. Which eventually gets you the spend the night watching porn scene. That's unlocks the relationship, and allows you to choose where yall want it to go. Which is Friends with benefits, or Dating. You can then choose between Monog, Open, and Poly dating (If you're poly). If you're not poly find someone that is and start dating them. Agree to a poly relationship, and you'll unlock the polyamorous Inclination.