First off, the federal government has no rights to what transpires in the bedroom. This should be a state issue. The same way one state can ban guns while others allow it.
2nd….based on the logic that equality includes all, incest and polygamy should also be permitted based on the notion as adults we are free to love whomever our hearts desires. The only rationale was based on religious law that forbids these marriages so now that this no longer applies, the other types of marriages should be allowed. Especially if I want to marry my dog Bernard as we are so close and love each other.