Substitute homosexual for black and the argument is absolutely clear.
I don't think 'homosexual' and 'black' are absolutely interchangeable in this context. It is clearly wrong to discriminate against people for what they are. The traditional Christian teaching is not that gay people should be discriminated against for what they are (homosexually orientated) but that homosexual acts are wrong.
As I've said before, I have difficulties with this position, although I am a Christian. It implies that people with a homosexual orientation are. through no fault of their own, barred from giving sexual expression to a loving relationship with another person, which seems cruel. Jon thinks the answer is to try to change people's orientation. But I doubt whether that is possible in more than a very few cases, and, even if it is, I find the idea rather distasteful.