Estate attorneys will hate this. Once everybody can marry anybody(s) they can marry their heirs (even their children) to avoid wills and probate.
I'd be willing to bet that the way the law will handle this will be to remove any and all tax and estate benefits for marriage and family relationships. Heck, you may not be able to file taxes jointly. The concept of dependents may go out the window. In our Brave New World such things may get too complicated, and so those benefits will just go away.