Images of doors, entryways, and doorways feature prominently throughout Willow and Tara's relationship, but Tara in particular has a lot of such imagery associated with her starting from her first appearance in "Hush." Indeed, Tara choosing to delve into the Scoobies' world of vampires, demons, and monsters is marked by a door.As Tara leaves her dorm room to find Willow so that they can do a spell together, she opens the door and looks back hesitantly at her room before shutting the door behind her.Tara's role for much of season four is allowing Willow access to her room – access to queer space – quite literally opening her door so that their relationship may foster.
Liminal spaces often play a significant role in portraying queer relationships.
Because same-sex relationships have been considered socially deviant, they oftentimes can only safely exist in liminal spaces, like a darkened alley or a bathroom stall.
Willow had been in Tara's room before to do magic together in "A New Man," but that scene begins with Willow already inside the room.
Thus, this little moment of Willow asking if she can enter Tara's room seems more significant than her simply inquiring if Tara wants to "do something." Their body language also suggests something more: Willow is visibly nervous and hopeful, and Tara's smile is on the warmer side of friendly as she lets Willow into her room.
After making that choice and thereby establishing her queer identity, Willow has freer access to Tara's (queer) space and no longer has to pause in the liminal space of the doorway.
Indeed, the next time Willow enters Tara's room in “Family” she opens the door without knocking.
But restricting queer characters to liminal spaces ensures that their "threatening" sexuality does not come in contact with "moral," heterosexual spaces like marriage, the home, and the nuclear family.
The confinement also implies that they do not or cannot belong in those places.
Combined with the door closing, leaving the audience outside the room, I'm inclined to believe that this episode marks when Willow and Tara's relationship becomes more than just a friendship.
This scene perhaps represents Willow's coming out to herself, choosing to enter Tara's room in a more significant way than before.
Or perhaps that relationship is actually inverse: because Tara is queer she must inhabit separate space, which makes her an outsider.