I agree that you should never get a guy in trouble with his lady. That is simply unconscionable. You don't dig around in his tools, and you don't drink his beer until offered one. Standard rules that we all obey.
This little blurb makes me call Bullshit
No matter what, you keep your grubby mitts off a woman that he has touched. That offense is punishable by him slowly crushing your man-sack in a vice-grip.
The actual application is that you shouldn't get caught with your grubby mitts on someone else's woman. But, lets say you are a college sophomore and and an attractive coed tells you that her boyfriend is going to be out of town for a couple of days and she is really lonely at the apartment, and would you come over tonite after dark and park around the block so no one sees your car.
Sure! I'd be happy to. Nothing at all wrong with consoling a lonely lady, as long as she keeps her mouth shut. I don't know the boyfriend, and if she wants to play around, it isn't any of my business.
The secret is never to bed a lady who has less to lose than you do. If she can lose a relationship, a marriage, a whatever, and she wants to keep that relationship, then the guy is safe. The reason for her wanting to keep a secret may be as simple as a really nice apartment that she wants to live in till the end of the semester. If boyfriend finds out, she is evicted. If boyfriend doesn't find out, she can stay. She has something to lose.
Never bed a woman who has less to lose than you do.