What about freedom of religion? Should a landlord be prohibited from exercising His God-given right to refuse to rent according to dictates of his consciensce? Those who answer in the affirmative don't have the slightest clue as to the meaning of freedom of religion(or private property rights, either).
It's not a question of "should?" It's a question of "do?" And the answer is "no."
Look at the Boy Scouts. The only reason they got out of letting gays be Scoutmasters is that they sold the Court on the notion that they were a religious organization. A commercial enterprise, though, is not a religious organization, even if the boss is religious.
As an aside, it was a pretty dang close case even with the Boy Scouts. A 5-4 decision.