Why is the word “prostitute” used to judge or demean women?

So, my friend and I were chatting about something, and he started talking about a girl. Usually, I don’t like talking about people - it’s not my cup of tea - but since he was already in the flow, I felt it would be rude to interrupt. He said a few things, like how the girl goes on new dates almost every week. What really caught me off guard was when he called her a prostitute. I found that extremely insulting and left the place.

This made me wonder: as a guy, why is the word “prostitution” used as a derogatory term specifically against women?

When I was in India, I went to Mumbai for my US visa interview and also visited my ex-girlfriend at her home. On my way back, at Bandra station, I saw a woman lying on the floor, badly abused. I went up to her, but she was scared at first - she thought I might beat her too. Apparently, she was a disabled beggar (she couldn’t walk properly), and someone had beaten her and taken her money. It was a winter night in Mumbai, so I gave her my jacket and some food.

While talking to her, I learned that she was originally from Uttar Pradesh. She had loved a man, and they ran away together to Mumbai. Later, her parents died by drinking poison due to shame - it seemed they belonged to a very conservative and poor family in a village in UP.

Her husband eventually pushed her into prostitution. If she refused, she was abused. He beat her so badly that she became partially disabled and couldn’t walk properly.

So my question is this: why is the word “prostitution” used to put women down or judge their character? (the discussion is not about a girl going on multiple date but the word prostitution, i wanted to make it clear) There are many prostitutes who are not in that situation by choice, but because they are helpless or were forced into it. Life is not the same for everyone.

What is your view on this?

TL;DR:

A friend used the word “prostitute” to insult a woman for dating often, which bothered me. I’ve met a woman who was forced into prostitution due to abuse and had no choice. Many women are pushed into it by circumstances, not desire. So using this word to judge a woman’s character is unfair.