No. It's an old thing where there are laws against housing discrimination. The estate agent would probably be on very shaky legal territory if they continued to list this property knowing that the owner had a 'no gays' rule.

As far as anyone being 'forced' to do something is concerned, you might want to note that no legal action has been taken in this case. The estate agent has simply chosen not to continue listing the property. Presumably you are not in favor of estate agents being forced to list properties that they don't want to list.