I'm going to use sexism as an example but the same can be applied to any other -ism you want or homophobia.
The main problem with Positive Discrimination is that it's just sexist. No longuer a person is a person, you have two different kind of people, the weak women that can't defend themselves in equality and the men. You are forced to be defined profesionally by your sex not by your skills.
It's patronizing and leads nowhere because you are not making women better, you are making them more "woman" but not more female but more genderized, you separate them from the men as something different.
Remember Luther King?
"I have a dream that my four little children will one day live in a nation where they will not be judged by the color of their skin, but by the content of their character."
What is Positive Discrimination but race over character? If you want meritocracy make meritocracy work, don't make an absurd rule that goes against meritocracy to make it better.
And, more importantly. Positive Discrimination is useless because it doesn't solve any problems. The problem is not "There are very few female engineers", the problem is "Women are being driven away from engineering because of they are female". Equality is not equality of resulst but equality of opportunity. All the girls in my class got promoted while hardly 2 o3 of the boys did the same not because they were discriminated against but because they were worse at studying.
The result was unequal, but it was fair because the important part of equality was the part when they all got the same test and got the same marks without any regard to their sex. An "unequal" result doesn't means that the equality of opportunity is broken or that it is broken in that stage.
For example. There are very few female engineers. The reason is not that the field is sexist, that girls are prevented from working there or a combination of the above but the main reason is that there are very few girls studying that field. In my class the ratio is close to 10%. Was it because they had a hard time entering the school? No, it wasn't. In fact the girls have on averange, far better marks than boys.
So, why there are so few women in the field? The answer is sexism yes, but not in the field per se, the answer are gender roles.
I mean, you can't expect to have a lot of female engineers if you don't have any girls studying it and you can't expect them to study it if they don't want to.
If we fired everyone from their jobs and pressed the magical no-sexism button we would still have gender splits like the one we have today.
Does that mean that sexism doesn't exist? No, not at all. All I'm saying is that it is not as overt and as directly responsable of the gender splits and the most important sexism is but gender roles and not grumpy men who refuse to let women do a "man's job". There are lots of those grumpy men, but they are not the main problem.
And even if they were you are better off fighting that instead of circunventing it in a poor way. Positive Discrimination will have very sexist sideffects on feminism and will drive lots of people from the notion that men and woman are equal.
People are not going to follow you when with one hand you talk about how men are women are equal and sex is irrelevant while with the other you talk about how they are not equal and women must have priority because sex matters. You only give the Strawman a point.
And someone who got a job because of this isn't going to be respected because she didn't earn it. She was a subpar option when it comes to what really matters, the job. People will complain and say "Meh, she only got here because of PD, she sucks" even if she worked hard to get there. You tain forever the reputation of female workers and you will give them facts to prove how bad women are.
Those effects would create a whole wave of sexism because now the sexist people are not going to have to admit just a woman but an unskilled woman, perpetuating the cliche that women are useless and that a man would do the job better.
I'm getting into a long rant so I will leave it there. Feel free to comment.