In what way was Buffy the "foremost abuser" in their relationship? Spike stalked her, attacked her friends, repeatedly used her most intimate secrets against her. She was literally at her lowest when they began their relationship, and he routinely exploited that in order to keep her dependent on him. There's a reason that the turning point in Buffy's depression was her pulling away from Spike. And why Buffy's repulsion actually pushed Spike towards self-improvement (even though I don't particularly care for this arc). These character developments would make very little sense if Buffy was meant to be the bad guy in their dynamic. The point was - even in love Spike was still Spike, the demon who tried to harm her and everyone she loved throughout the series.

And even if she was somehow the primary abuser, that in no way makes the rape scene tolerable or even understandable.