Ken Paxton took a serious oath when he became the Texas attorney general.

He put his hand on a Bible and vowed to preserve, protect and defend the Constitution of the United States and of the state.

Then the U.S. Supreme Court did something Paxton — I presume — didn’t expect. It ruled that gay marriage was legal in all 50 states. All of ’em. Including Texas.

How did Paxton react? He said county clerks weren’t bound by the court ruling, that they could refuse to issue marriage licenses to gay couples if the issuance of such documents violated their religious beliefs.

Oops! Can’t do that, said the State Bar of Texas.

It’s now going to launch an ethics investigation to see if Paxton — who’s already been indicted for securities fraud by a Collin County grand jury — violated his oath.

Well, of course he did!

If I were able to make a call on this, I’d declare that the AG broke faith with the oath he took. So did that county clerk in Kentucky, Kim Clark, who refused to issue marriage licenses to gay couples and who spent some time in jail because of that refusal.

What I can’t quite fathom is how these elected public officials feel they can get away with refusing to serve all their constituents. Paxton is a statewide officeholder, representing 26 million Texans. He won election in 2014 and then swore to follow the laws of the land. Not just those with which he agrees.

The Texas bar would seem to have an easy decision on its hands as it regards whether Paxton violated his oath of office. The tougher decision will be in the sanction it should level against him.

I am not going to say he should be removed from office.

Honestly, though, it baffles me constantly that these public officials — who get paid to represent every constituent — think they can select which laws to obey and which laws to flout.

That oath is clear. They cannot make that choice.

At all.