Perhaps I was a little to definitive in my response. Real estate law may vary by state, but real estate agents are forbidden from discriminating against gays in rental or property transactions under various laws. Could an individual renting or selling their home or property discriminate, perhaps in some states.
As to employment discrimination against gays, it appears this is handled at the state and local level, while federal government hiring has interpreted (or extrapolated)
that they can't discriminate against gays under the existing discrimination laws.
Interesting. It may be that real estates are forbidden to discriminate by some sort of ethical or professional code, but I don’t know of any federal law to that effect.