I've run across this several time now in Buffy fandom, when Buffy is called a slut for sleeping with Spike. And I'm quite sick of having to defend her, while the male cast is never accused of slutty behavior. Xander has been with almost every heterosexual woman on the show and only rarely was it the result of teh great love. Angel slept with women, he didn't really love, so did Spike and so did Wes. Yet somehow it's always the girls that end up with that accusation.
And as if that wasn't enough, now people seem to pile on Rita on Dexter, for getting kissed by that neighbor guy. It's just grrrr!
And double grrr for how many women in fandom seem to push this saint/whore-stereotype.