Basing morality on the subjectivity of an individual’s happiness causes many problems. For one, it always will leave a minority view open to oppression. Because if the majority find happiness in the way things are, we can never overrule that.

For the sake of argument:
If the majority view, democratically instituted, was that happiness in sexual relations culminates in natural child bearing and rearing, and all other forms of sexual relations, which are not for happiness but rather lewdness, we shouldn’t allow any other sexual relation. If the majority finds happiness in it, why should the minority overrule it?