Anti-discrimination laws seem like a great idea, right? For example, in many jurisdictions it is unlawful to discriminate based on marital status. So that way no employer can have a policy 'we don't hire married women' or 'we don't hire single women'. That's a good thing, right? Can't possibly have a down side.
Except it does. If the anti-discrimination law is very broad--- perhaps simply stating 'discrimination based on marital status shall be unlawful', with no further clarifications and restrictions--- it can have many unintended effects.
For example, adultery. Adultery is a destructive force, the cause of divorces and violence. But with a broad anti-discrimination law, a motel owner may not refuse to rent a room to a married man and his mistress. That would be discriminating against the adulterer based on his marital status.
And what about an open, public adulterer who applies for a job at a family-oriented business? If he is turned down, he can claim discrimination based on his marital status, since being married is a prerequisite for adultery.
The laws against adultery serve an important purpose to society. Faithfulness in marriage makes marriages stronger and prevents divorce, which in turn reduces the risk of childhood poverty. Faithfulness reduces the spread of social diseases. Faithfulness in wives ensures that the children born to those wives are not the biological children of some outside lover, but of the husband. But sweeping anti-discrimination laws make it impossible to take note of the fact of adultery, and weakens the social sanctions against it.
This is why proposed anti-discrimination laws must be very specific, to ensure that no unintended effects will result.