Faith is fucking Dawn, but it's all about Buffy.
Since Faith met the blonde Slayer, it's always been about Buffy.
Dawn probably knows, she's not a stupid kid, and Faith figures it's probably about Buffy for her too.
Faith isn't using Dawn as a substitute; Dawn isn't really like Buffy anyway, physically or otherwise. But she is probably the closest Faith will ever get to fucking Buffy, and the thought makes her thrust angrily, and the deep moan Dawn gives in response drills in with no uncertainty that this is not Buffy.
Buffy would whimper, she thinks. Maybe put up a bit of a fight, as opposed to Dawn eagerly opening her thighs and allowing Faith to do as she pleases. She gives another angry thrust. There's another moan.
Faith is doing this with every intention of Buffy finding out. She is planning on still being on top of Dawn, inside Dawn, when Buffy opens the door in a few minutes. And when Buffy sees Dawn, freshly fucked, and begins to yell and scream, Faith can plead innocent because Dawn was more than willing in this, and she can walk out with probably nothing more than a split lip, and it will be worth it.
Because this will crush Buffy, and she'll know that Faith did it because of her, and Faith isn't sure what Buffy will think about Dawn but she doesn't think it will be good.
Then Dawn grips her shoulders, so tight, but Faith doesn't feel it, or hear Dawn's deep gasp, because the door is swinging open.
She smiles, and waits for the first punch.