Seawytch
Information isnt Advocacy
If they are adults it isn't a crime. It might not suit you but two of the same gender don't suit others. Gays can't have kids so what is your good secular argument if one or both relatives are sterilized? They're in the same boat. Maybe it's a mother and daughter of age. What's the good secular argument against polygamy? You are simply imposing your moral values onto others while condemning the imposition of the majority's moral values. It makes no sense.The fact that you like to have sex with your mother and daughters is your own business. Though you might be confessing to a crime, so I'd be careful.
However, there are good, secular reasons to forbid incest, and polygamy, while there are no longer good secular reasons against homosexuality or same sex marriage
You are wrong. In most states it is still a crime. Laws regarding incest - Wikipedia the free encyclopedia
In the United States the District of Columbia and every state have some form of codified incest prohibition. In all states, incest is sexual acivity between a lineal ancestor and a lineal descendant (parent, grandparent with child or grandchild), siblings (brother-sister) and aunt-nephew, uncle-niece. However, individual statutes vary widely. Rhode Island has repealed its criminal incest statute[66] and only criminalizes incestuous marriage.[67] Ohio "targets only parental figures",[66] and New Jersey does not apply any penalties when both parties are 18 resp. 16 years of age or older
You are also wrong in stating gays can't have children. We're gay, not infertile.