Incest will have to be legally recognized.
There's only one secular reason to legally forbid incest. And the reason is that offspring produced from such a union have a greater likelihood of being deformed. (The actual numbers of deformities from babies produced from an incestuous relationship as opposed to non-incestuous relationship are only slightly elevated.)
With same sex marriage, procreation is taken off the table. That's not my opinion, that's biology 101.
So, what would be a secular reason to prevent two adult sisters from getting married? Or how about two adult brothers? Or how about if we dig further into the sewer of leftist Utopianism and ponder why a father/adult son couple would be denied.
Think about how weird this would be. You would have a situation where a father/son couple could get married, but not two first cousins of the opposite sex. A brother and sister could not get married, but two brothers or two sisters could.
And of course, the satanic left will act completely surprised, but it's all part of their plan to drag down the human race to the level of animals.
There’s no test in law that require sexual activity between partners, so there shouldn’t be any prohibition against related same-sex siblings to marry. Then later, parents could “marry” their children and grandchildren and avoid the nastiness of wills and probate altogether.
On the other hand, plenty of people are well aware of the increased risk of having genetically defective children as a result of incest and therefore avoid incest, or at least get heavy on birth control to be sure that their behavior doesn’t produce a child.