Interesting stuff. I only use the term "slut" when I know someone's inner workings and personal history. It's entirely situational. If a women's sexual activities disgust me on a personal level, then they get the label "slut". My personal sexual put-down for a man is to call him a "skeevy/shifty ************/pervert".
While I know that this can be personally biased, I think there's a huge difference between a woman who's comfortable talking/engaging in sex (or dressing provocatively) and someone that I find morally reprehensible. I understand how important healthy sexual activity is to human beings. It's when people make it either unhealthy or completely trivial that I get offended.
Example of what I use the term "slut" for. I have an acquaintance (a women) who's at least had over 25 sexual partners. She had a child as a result, which she doesn't give the proper care to (essentially, she lets her parents take care of it while she goes to get smashed at bars and pick up guys). In hearing from a couple friends who made the mistake of sleeping with her, she apparently doesn't use protection (though, thankfully, my friends insisted). One night, she came over to me and my friends while we were having a pint, pointed out two guys that she was with and asked which she should sleep with that night. I, disgusted that she is making sex so trivial that she's asking outside opinions from guys she barely knows, said sarcastically, "why don't you just flip a coin?". And she did...it was heads.
I definitely use the term "slut" in referring to her. She's disgusting and deserves a title that carries my feelings of disgust towards her.