Which reference do you go to for that?
I go mostly to the teaching that Jesus gave in the gospels:
“You have heard that it was said to those of old,You shall not commit adultery. But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart. If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.” (Matthew 5)
Here we are told that to commit adultery even with our eyes makes us hellbound, it is better to be sightless; a pretty strong condemnation.
Then if you search against all the warnings against adultery as the soul destroying activity that it is, this just undergirds the warning.
I find the plea in Proverbs 5 to be especially poignant:
” My son, keep your fathers command,
And do not forsake the law of your mother.
Bind them continually upon your heart;
Tie them around your neck.
When you roam, they[b] will lead you;
When you sleep, they will keep you;
And when you awake, they will speak with you.
For the commandment is a lamp,
And the law a light;
Reproofs of instruction are the way of life,
To keep you from the evil woman,
From the flattering tongue of a seductress.
Do not lust after her beauty in your heart,
Nor let her allure you with her eyelids.
For by means of a harlot
A man is reduced to a crust of bread;
And an adulteress[c] will prey upon his precious life.
Can a man take fire to his bosom,
And his clothes not be burned?
Can one walk on hot coals,
And his feet not be seared?
So is he who goes in to his neighbors wife;
Whoever touches her shall not be innocent.
People do not despise a thief
If he steals to satisfy himself when he is starving.
Yet when he is found, he must restore sevenfold;
He may have to give up all the substance of his house.
Whoever commits adultery with a woman lacks understanding;
He who does so destroys his own soul.