Evolutionary reasons, as almost anything in human behaviour.
Society with developed degradation of easy women guarantee higher probability of faithfulness of such women.
Basically, cheated is more difficult, because historically, once a woman gets reputation of a slut, most men do not want to have serious relationship with her.
(This is also behaviour conditioned by evolution.)
Naturally, women want to have serious relationships and do not want to be spat at on the streets. This forces them being more faithful (or more careful when cheating).
The point is, that in a slut shaming society there is smaller chance of men raising somebody else's child. Which is making such behaviour an evolutionary advantage.
In a hypothetical society (regardless of the size, can be a nation or a family) where slut shaming isn't developed there is a higher chance of men raising other men's child, due to the fact that in such society cheating is easier and is not a social stigma.
Therefore none or limited slut shaming is an evolutionary disadvantage.
As for using a word slut, yep, I am using it.
Don't care about political correctness and other crap.
Languages developed in a certain way, and trying to artificially change the way people use their own language smells too much like 1984.
Therefore, doctor is a doctor because he/she does what doctors do.
And slut is a slut, because she does what sluts do.
BTW, when I use the word slut I use it either in a neutral sense (simply describing what she is, see the example with the doctor), or as a compliment.
Nothing is better than an independent woman enjoying her life
