Why do people call young girls who hhave had sex names?

Hi, I have asked a question on this before an loved some of the answers. My last question was why people are so judgemental of girls who have sex at quite a young age. No I was just wondering why it is that you would call someone a slut for having sex with a long term boyfriend or without really knowing any of the details of how and why it happened? My personal opinion is that it's harsh to call a girl a skank and telling her to go to hell when you know nothing aboutthe situation. This is just my opinion and I don't mean to offend or tell people what they believe is wrong, I'm just interested in your opinions as well. Lastly I just wanted to add that I would never call someone who is saving themselves for marriage or a virgin frigid or naive as I respect that is what they believe, so why do is it ok to call people who don't think that whores and sluts? Thank you.