but you are not forced to be in a "relationship" with them. It's up to you how far you develop with them. otherwise, they fill as much part of the story as any other character.
even if you are not Sage fuck buddy, She will call you when she break up with Chad, she will ask you to visit and help her when she is sick. rest assure you, you also will have to deal with Jill and Tybalt, even if you not chasing Jill.
but the game not force you to be on romantic path with anyone , , It's just opens the possibility the rest is up to you