Hebrews 13:4 Marriage is honourable in all, and the bed undefiled: but whoremongers and adulterers God will judge.
I just kind of think that scripture is appropriate for your question.
I can only comment on what I think. I strongly disagree with a Christian's use of pornography. I believe it will cause a person to lust, is of the flesh, and can lead to addictions. Also I've read studies on where porn users will often seek "more perverse" material in order to get that same "high" off pornography.
Cheating..... (my opinion only), I believe the word cheating would be a strong. I do believe most women would be hurt if they walked in and their husband was looking at pornography and defiling himself with it. However, if she were to walk in and another woman was in the room with him.... You get the point I'm sure. I think most women would be far more hurt with a physical woman present.
I would see it more like "violating trust", "seeking the flesh", and adultery in his heart. I believe it would leave a wife thinking they are inadequate for their husband and inferior to the young woman often depicted. It would hurt her self image very badly that she could not satisfy her husband and he turns to pornography.
For the man it can hurt him because he will be trying to gain physical satisfaction from a digital image of (usually) a young woman. When a man releases, it also releases dopamine in his brain. This will form a bonding (through a short period of habit) to the form of release he is using. If the man is seeking out pornography for his release, he will eventually be hooked on that release. (There's several studies on this).
This is how pornography addiction works.
On the flip side, if a man were to love his own wife and admire her beauty as is written in the song of songs, then he will be of one flesh through matrimony with his wife. Through this mutual bonding can occur. Dopamine will also be released in his brain and likewise his wife's as well. This can form strong bonds between a husband and wife. Through the blessing of marriage, the blessing of a physical relationship, God often blesses them with children. Children growing up in a household with two parents who are closely bonded are blessed.
I do agree with what you say of adultery in his heart. I suppose the question would resonate - is adultery in the heart the same thing as adultery itself (of the heart and flesh?) I am not sure.
I do think most would agree however that pornography is very damaging to a person. Is it the same as walking in on a physical "cheat" of a spouse???
On a side note, being in the IT business so long, I'll tell you..... Pornography is so incredibly "common place". At first it wasn't like this. But so many of these young men are really hooked on this stuff. May God help this world.