Is there a law in Washington state against adultery?

From: Adultery - By Franklin Foer - Slate Magazine

In fact 26 states have laws against adultery. I don't know if Washington does or not, but to quote the article:

"In practice, adultery laws matter little: Only one case--against an Alabama man--has been prosecuted in the last five years. Most states have not enforced their adultery laws since World War II"

So it's a moot question.

In Washington state does adultery get considered in where young kids live in divorce cases when it is a same sex relationship?

In Washington state, adultery does not factor in, in matters of property or custody. See:
Divorce In Washington State
Scroll down to the section called, "Residency Requirements and Ground for Divorce"

Now there is a private investigator agency in Washington, called, "The Gilbertson Group". They do claim that adultery can be factor in making a bigger settlement. Read this from their website:
"Infidelity & Adultery
Providing clients with evidence of infidelity or adultery can assist them in achieving a more lucrative divorce settlement. Evidence proves Inclination and Opportunity. An example of Inclination is two people holding hands, kissing, or showing affection towards one another. An example of Opportunity is two people entering a hotel, residence, or other private location together.

We'll discover the truth by acquiring evidence. Evidence is the currency of our justice system. Our evidence will prove infidelity, and may help secure your financial future. Even in "no-fault" divorce states, proof of infidelity can be a powerful bargaining tool in your settlement negotiations.

Photographs, video, and witness affidavits are just some examples of the proof we can acquire. When confronted with the evidence against them, your spouse will likely settle your case and save everyone the expense and embarassment of a public trial. "

This is their website: Seattle Adultery and Washington State Adultery THE GILBERTSON GROUP

Now before Washington became a "no fault" divorce state, adultery was considered one of the grounds for marriage dissolution. That was thrown out in 1973, I believe. You can read through the "new" legislation that was enacted concerning the no fault divorce at:
Washington Bill Allowing Contracts to Restrict No-Fault Divorce

Is it against the "LAW" or not? If it's is and not being "enforced", than "take it off the books".

Why have a "Law" if someone picks & chooses which ones to enforce?

Hipocrites?