Treating variables as constants

The case for zoning laws which prohibit building entirely because they aren't aesthetically pleasing is parallel to the case against anti-discrimination laws. The reasons to support one are structurally the same as the reason to oppose the others. The major justification for not allowing a house to built because it is ugly is that it will lower the property values of the surrounding houses, imposing an illegitimate externality. But this takes market valuations of property values as constant, something which won't be changed as a house previously considered ugly becomes neutral, or even attractive. But since such laws don't allow the houses to be built, they never have a chance to become prevalent and the market valuation doesn't change.

The case for allowing employment discrimination in (for instance) law firms. The major justification would be that it would lower the rates your firm could charge to others, even for the employees who aren't members of a discriminated against class. If the marketplace values the work less, your firm is losing value by being forced to not discriminate. Again, this treats the valuation of the market place as a constant, rather than something that can be changed. So employing lawyers who are members of a discriminated against class may well cause the market to become neutral or positive towards these workers

Which isn't to see these zoning laws are as invidious as employment discrimination, since that hurts a more valuable interest, but just that zoning laws based purely on aesthetics are dangerously unjustified.