What would happen if trump were to issue an executive order banning same sex marriage and the supreme courts ruling on same sex marriage?

Nothing would happen. Same sex marriage would continue. The order wouldn't do anything.

Law-abiding states would continue to perform them. Hick hillbilly Trumpistan states might jump at the chance not to.

why would trump do that ?

trump isn't a homphobic bigot like the democrats elected in 2008

He does not have the power to overrule the Supreme Court.

Once again, the Supreme court issues opinions, not orders. Ask Andrew Jackson about this if you doubt me. 

Good question.

It is the job of the supreme Court to INTERPRET THE LAW and so their ruling is final.

If a President gives an executive order countermanding their final decision, he is out of line and lower courts will readily overrule any challenge to what the Supreme Court has all ready decided.

In short, the president looks like a fool because he is overstepping his bounds and going against the final ruling of our US Supreme Court is NOT his job.

Each branch of our government has a role to play in our government.  And so in the same way the Supreme Court cannot issue an "executive order", the president cannot countermand their rulings on constitutional law. 

The SCOTUS doesn't work for the president.

An executive order cannot supersede Constitutional law, and SCOTUS has already  interpreted Constitutional law as allowing same-sex marriage (Obergefell v Hodges). 

So his order would have no force.

Trump is not against those things

"What if", we stick to reality. He has no intention of doing anything like that.