The anti-discrimination laws already apply to some groups and not others; the Civil Rights Act of 1964 (you should go read it) protects against discrimination based on race, color, sex, national origin, or religion. So theoretically, it's perfectly legal under that statute to discriminate against gays. Sexual orientation isn't protected. So you see why a law would need to be passed protecting that group explicitly, unless they want to say "it is against the law to discriminate on the basis of membership in any group", but then you can get some ridiculous interpretations like "you can't throw me out of your store for destroying the displays because I am a member of the group of people that likes to destroy things in stores."
Well yea. You can kill anyone you want too... I didn't say there wouldn't be consequences.