Sex with your Best Friend is opened up by 'completing' their relationship storyline. So far as I know, that is the only way to accomplish it. It's not quick. I think my 'speed run' was like basically the entier first 'semester'. Part of this is because it also involves the 'Friday night Party Secret', you'll know it when you encounter it & you have to bring that to a close as part of bringing the relationship to a climax. Besides that aspect, you also need to choose the 'Sex Talk' option whenever it comes available when hanging out with them. It only becomes available occasionally as it also requires that your best friend's roommate not be there when you go to visit. you can talk with your best friend in their dorm once a night, every night except Friday, between six & ten.