Most of the characters on BtVS, especially the male characters, put Buffy on the pedestal of the perfect hero. With the two vampires who remade themselves into heroes for her, it goes beyond simply seeing her as a heroic, to seeing her as representing goodness. Someone once put it that way: in falling in love with Buffy, Spike fell in love with goodness itself.
It's hard not to see that as gendered. Women, more than men, are used to represent all that is pure, and right, and good in the world. On many shows (BtVS being a rare exception), the heart of the team is a woman. Women are compassionate. Women are oriented toward others. A woman can represent goodness.
Of course, the flipside of the woman who represents Goodness is the woman who represents Badness. In S1, Buffy represents Angel’s good side while Darla is his bad side. Drusilla takes Darla’s role in S2, and Faith takes the role in S3. There is a bit of a subversion with Faith--the surface love triangle of Faith and Buffy competing over Angel (both romantically, and over whether he should be bad or good) is not the more important love triangle. Instead, the more important love triangle is the below-the-surface triangle of Faith competing with Angel for Buffy, who she wants to corrupt. Unlike Darla (whom Buffy only has one conversation with that I recall, and of course it is about Angel) and Drusilla (with whom Buffy never really has a conversation with, despite sharing two boyfriends and encountering each other several times), Faith has a relationship with Buffy that is separate from, and more important than, their competition over Angel.
Drusilla takes the role of the woman who represents badness in S5 and S7 for Spike, instead of Angel. Over on AtS, Lilah represents badness for awhile Wesley, while Fred represents goodness. There’s a rare genderswapped version in Buffy S6: Spike represents badness, Riley goodness. But here’s the thing: Unlike Lilah, Dru, Faith, and Darla, Spike isn’t actively doing evil. This is mainly due to Buffy’s personality: Even being involved with a guy who has no remorse about the evil he did in the past freaks her out. If Spike were actively doing actual evil, Buffy would kill him. If he were human, she wouldn’t kill him, but she certainly wouldn’t sleep with him.
One the things I really like about the Buffy-Spike-Wood-Faith dynamic in S7 is that, although loyalties are very much tied to flirtations and sexual partnerships, it’s never a bad-girl-vs-good-girl thing, or even the gender swapped version. It’s more complicated than that.