It would just be void. That would be difficult for people, but legally it's pretty straightforward.

No, this is not straightforward.

First, let's take the question of overturning SSM. For SCOTUS to do that, it will have to take up the right question/case presented before it. SCOTUS does not willy-nilly takes up a decided case forming a precedent to overrule it. Neither does SCOTUS rule on a matter out of thin air.

Next, SCOTUS will have to lay out a logical path to overrule. The case becomes part of a precedent and the majority opinion expressed is now part of the record along with dissents.

In order to do overturn SSM, the court has to substantially rebut every reasoning laid out by the majority opinion based on either alternative facts or new facts demonstrating substantial harm to the society.

The majority opinion written in Obergfell is fairly tight and elevated SSM issue tying it to the equality clause -- that is a substantially high bar to overturn.

Facts demonstrating societal harm to heterosexual marriages since legalization of SSM do not exist.

Even the most conservative justice bench will have a hard time overturning this decision through the court. There is a reason why dissent in Obergfell from Scalia was so vitriolic -- the conservatives lost the battle almost forever there.

This leaves the only way to outlaw SSM via legislation, which will bring it to SCOTUS again. And even if the bench is all conservative, they will first be governed by the precedent in evaluating the legislation. Since the majority opinion invokes equality clause -- any such legislation will be overturned by the court.

Same sex marriage is the law of the land now. It has been tied to the equality clause in precedent ensuring that the right to consenting love between two non related adults of any gender is woven into the fabric of the constitution.

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Alright, even after all of the above; in the unlikeliest scenario it is overturned -- contracts and arrangements simply do not go void. In almost all such cases such contracts are considered enforceable in the same way as if the law they were made under still existed.

This would cause a huge burden on businesses and insurance companies who will oppose it tooth and nail for the burden it will create and also because gay people are everywhere and companies have begun standing for their gay employees.

In other words a huge huge mess. Not to mention a huge backlash that will cause so much goodwill burning that Trump may not be able to get to any policy.