There are no rights protecting illegal acts.Monogamous and polygamous marriages are two different entities. If you legalize monogamy, it does not require that you legalize polygamy.
But if you legalize monogamy, you can't then deny it to same sex couples without discriminating against a minority.
Actually there are rights that make acts illegal. Discriminating against people of a certain race for example, was made illegal because of rights for those people coming from the Constitution.
Illegal acts are not protected forms of rights.