Religion is also a protected category, for what it's worth. Nevertheless, many people disagree with allowing a religious loophole to discriminate against gay customers.
I know it's more complicated in practice because the problem is political and other factors come into play.
If a Muslim man is punished by his employer for not shaking hands with women he meets in his work, is that religious discrimination? Or is he discriminating against the women because of their sex (also a protected class)?