Only prudes believe in sluttiness.
Honestly though, my opinion of what's "excessively promiscuous" depends on the situation - a person who has slept with the majority of a single social group would probably seem slutty to me, whereas someone who had slept with the same number of people in the same timeframe but weren't a part of the same group would not seem to be (so to analyze myself, I suppose I have problems with people who mess with social contracts, as someone who sleeps with an entire group of friends likely would be doing). Also, sleeping with multiple people in separate instances on the same day pretty much guarantees ones sluttiness.
In regard to why sexual promiscuity is a bigger deal (to me) than other types of excessive behaviour, the issue is twofold: one, it is nice to feel at least a tiny bit "special" when in the position of having sexual relations with someone (even the smallest bit, like being the person they're most attracted to out of a nightclub full of people) rather than just being a glorified masturbatory aid, and the second part being that, as safe as one attempts to be, the more partners one engages with, the greater the likelihood of picking up an STD. The latter part is generally not a major concern, but it still exists on the periphery.