Its not widely known, but part of the review process for a security clearance is to see if a person has any history that could be used against them as leverage. Large debts, gambling habits, affairs, unsavory/embarrassing sexual habits, etc. These are all taken as signs that either the person has poor judgment or impulse control, or can be manipulated. It can mean being denied a clearance, or losing an existing clearance.

An example of why the government takes security clearances so seriously is Aldrich Ames. He was compromised by heavy debts and his affairs.

In the modern day it is a big deal in the military because it shows a lack of character. That the person cannot be trusted. That they can be tempted or swayed, and their judgment is not good.

The military handles a lot of sensitive, secret equipment and information, and a soldier that makes bad decisions in their personal life can carry that into their professional life. It also provides an opportunity for espionage to leverage that against them. "I know you're having an affair, I'll keep quiet if you tell me how this secret device works, or bring me the manual for the new widget."

It also really fucks with unit cohesion, as soldiers deployed have to worry about whether their loved ones are having an affair with another soldier while they are gone. It is distracting and dangerous.

So the military takes it seriously, and enforces it as a crime.

There's also some historical reasons on it that have to do with honor and some religious underpinnings.

Pregnant soldier killed by another soldier. High likelihood that one or both of them were married. The US military takes adultery pretty seriously. If she intended to put his name on the birth certificate and file for child support, there's the potential for the dude to lose his security clearance, get charged with conduct unbecoming, and get discharged and lose his benefits.

He may have felt that murder was a better alternative crime to be charged with than the shit blizzard about to be dropped on him.

Still a monstrous thing to do.