It's important here to separate issues. One issue is whether there is something morally objectionable in a person's having multiple sexual partners. Another issue is whether the answer to that that question partly depends on the gender of the person involved. Feminist opposition to "slut shaming" has entirely to do with the double standard regarding sexual promiscuity that prevails in our culture: a cultural presumption that there is something more shameful about a woman's having multiple sexual partners than about a man's having multiple sexual partners. In many milieus, it accrues to a man's status for him to have multiple "conquests" to his credit, while it decrements a woman's reputation for her to have had sex with an equal number of men. Why should that be? How could promiscuity be morally different for a man than for a woman? The idea that there is such a moral difference is what feminists are objecting to. Doesn't that seem perfectly reasonable?
There are a number of interesting things to consider here -- for example, the idea that women, but not men, are somehow damaged or defiled by sexual intercourse. If it is revolting to consider having sex with a prostitute, who may have had dozens of different partners, why is it not equally revolting to imagine having sex with a male "player" who has "conquered" dozens of different women? Why, in fact, do we even have the word "slut" in our lexicon, but no matching word for a man who indiscriminately has sex with a large number of different women?