That is why it is used as an insult. (Along with words like “whore”, “bitch”, and “slag”.)
She is a slut if she steps out of line. She is a slut for being sexual. She is a slut for not being sexual with the man who desires her. She is a slut for being naked. She is a slut for being attractive. She is a slut for being free.
And only a woman can commit this offense.
Her entire worth is based on this judgment, with disregard for her loyalty, honesty, morals, and values. Every sexual act is subject to scrutiny. She is demonized for the same actions committed by others, only depending on the motivations of the accuser.