Re: What Is Wrong with "slut shaming"?
I will never call a woman a slut; it is indeed a terrible word. I will never try to shame her in any way, shape, or form.
This said, let's look at the issue of promiscuity - maybe even this term is too pejorative, but let's look at the behavior of having some sort of large number of ever-changing sexual partners.
I'll look at it in two different ways.
Situation A: The woman is wildly enjoying the sex and fulfilling herself, and she feels content and happy. She is using precautions to avoid STDs and PID (pelvic inflammatory disease). She initiates the sex and/or consents to the sex because she really wants it and likes it. She is able to refrain from it when the conditions are not right. She avoids hurting anybody else with her behavior (that is, she is not a serial cheater). She practices efficient contraception if she doesn't want kids at this point in her life. OK, these characteristics being true, I'll say, good for her! I'm happy for her that she is enjoying herself. If necessary, I'd try to put at her disposition resources to ensure that when she does it, she continues to do it in a safe and responsible manner, and/or she can have access to help if her situation were to change.
Situation B: The woman doesn't really enjoy the sex; she does it out of low self-esteem and insecurity, thinking that she is a good-for-nothing so in order to deserve any attention she needs to jump into bed with any guy who wants to screw her; then afterwards she feels guilty and dirty and with even lower self-esteem; she takes few precautions and ends up with PID and/or STDs; she feels forced into it by domineering and sadistic partners or does it in exchange for drugs of addiction that she has no power to self-control; she is a victim of sexual abuse and it twisted her sexuality in a way that she feels compelled to repeat it over and over; she is in a committed relationship but repeatedly cheats on her partner and at some point her behavior is discovered resulting in extreme pain for the partner and/or other people (like children); she has repeated unwanted pregnancies that result in serial abortions that end up being dangerous too (for example, if repeatedly done by poorly qualified people in less than sanitary conditions) or irresponsibly gives birth to unwanted children who then get neglected and/or abused. Well, if one or more of the above is/are the case, then, it's not good for her, and I will experience pity for her, and will try to offer help (counseling, medical and psychological treatments, social protections).
So, whether or not this behavior is pleasurable or detrimental, depends on the woman and on a large set of circumstances. It's not black-and-white. These circumstances are also not written in stone and can change. A woman in situation A for example can end up jaded and can start questioning her lifestyle and the pleasure equation may change; a woman in situation B can take some corrective action and start enjoying it in a healthier way... and everything in-between.
So, I don't look into it from the moral/judgmental standpoint. I look into it in terms of whether the woman is well and safe and not being harmed by her behavior, or she is not well, is getting harmed, and is in need of serious help.
This, by the way, applies to men as well. But they are being harmed and need help more rarely than women, because they count on more societal approval, and fewer risks (they won't get PID, they won't get pregnant, although they can get other people pregnant with disastrous consequences, and can get STDs).
This is not sexist in any way: it's just that if I approach the issue from the standpoint of finding out who needs help; men just need it less frequently, for various reasons. That's just a fact; I wish society didn't have double standards, but I can't change society. This said, even if less numerous, there are cases of men who do need serious help regarding sexual behavior, and boys/teenagers who need guidance to learn proper behaviors and precautions.
And to answer the OP's question, what is wrong with slut-shaming? Well, you won't accomplish any lasting changing in behavior by shaming people. Often you'll perpetuate the low-self-esteem problem, if shaming is your approach.