Discrimination in itself is neither good nor bad.
I will discriminate against a high school dropout vs a brain surgeon if I want to be diagnosed with this splitting headache.
Sorry, no, I'm not going to a high school dropout for medical treatment, no matter how you protest. You'd have to demonstrate some sort of miraculous skill for me to even consider you. But otherwise, no, nyet, never.
I will also discriminate against people with a tattoo of a pentagram/upside down cross with the devil on his forehead as a cashier to McDonalds. Sure, he might be awesome as cashier, but he's also likely to turn away customers. Not fair? Don't care.
Discrimination is bad when you judge anything as undesirable using criterion that is absolutely not connected to the topic at hand.
For example, you refuse to go to the black doctor because he is black. Or you refuse to hire (say, for a janitorial job, or accountant) a gay person. Being gay has nothing to do with his ability to perform a job.
...wait what. How does preventing discrimination innately discriminate?
Quotas. Affirmative action.
For example, a university has to admit 10% non-whites as freshman. So, if you're white, even if your entrance exam scores are higher, you won't be able to enter said university.
Thus, in an attempt to prevent discrimination, it actually causes more discrimination. You didn't enter that university SIMPLY BECAUSE YOU WERE WHITE.
This has gotten so bad that I've seen people in some forums claim that putting "caucasian/white" in your entrance form to a university means a greater chance of not being admitted.