“Harlot” is a word used to judge a women's value solely based on her sexual availability. To people who use that term, a woman is an object/toy and they don't want a used up old toy, they want a new one. So a woman who has been with a lot of men is seen to be worthless, regardless of whatever her personality/education level is.
Not only is “Harlot” an insult, it is based on sexist ideas about what a woman’s role is. The idea stems from the olden days when a woman’s life would solely based on servitude to a man, with no rights to luxuries, happiness or enjoyment for herself and to have freedom of choice of who she would sleep with would be considered “dirty” or “wrong”.
Some insecure women also call other women “Harlot” to give the impression their value/worth is higher than the woman they are referring to.
Sometimes men feel a sense of entitlement to a beautiful girl and if they can’t have her, it hurts their ego and the reaction is similar to that of a small child. Name calling such as “Harlot” to win the approval of other men with a similar sexist attitude behind their normal everyday lives. They resent women’s rights. They don’t want women to be equal, they want preferential treatment and wish women were tools for them to use and keep shut in the kitchen and the bedroom, even if they don’t say it out loud.