The roots of this are notable in that the basic design and layout of men and women are different and, as such, we acknowledge these differences in the most subconscious way. That is, there are things that we acknowledge that we do not consciously think of. And in this case, it revolves around gender roles.
I know I use lions metaphorically a lot, but it's applicable, so we're going to imagine if we were lions, yes? As the King of the Pride, we have a lot to choose from and the more active we are, the more desirable we are because our fertility is deemed healthy. Conversely, as a lioness, we realize that the more active we are, the more infertile we are presumed because of our natural body chemistry.
That still not doing it for you? No worries. I'll explain the subconscious human psyche next.
You see, without us realizing it, we have a set perception of how things should run. And one of the things that our brain -- no matter how much pondering and hypothesizing we do -- accepts as fact is that men and women are in competition.
Not so much in a side by side, dashing for the finish line manner. Rather, in a face to back, cat-and-mouse game. It is the ultimate goal of the man, the cat, to "catch" the woman and this is represented, symbolically, by the ultimate form of human expression. Id est, sex. Conversely, it is the ultimate goal of the woman, the mouse, to escape the cat and "win the game".
Confusing? It can be. But our chemical makeover pits the man as the hunter and the woman as its "prey". Philosophically, this may be a difficult concept to understand, but when you accept that not everything we perceive is through critical thinking, it becomes more apparent.
Basically, what this means is that society dubs a man who wins the hunt a "winner" and a woman who is caught in the hunt a "loser". Thus, the word "slut" is synonymous with "losing the game" and that is why it's viewed as taboo.