Because pornography use by men has the similar effect against pair-bonding that pre-marital sex has on women. We have been told by our priests that one use of porn can destroy a man's natural attraction for real women for life. That will ruin a marriage.
Take it from a former porn/self-abuse addict: the destruction of the man's natural attraction for women is NOT "for life". It bounces back within days and within months the porn-addled mindset, which usually causes fetishistic desires, is gone. Don't believe me? Look up the NoPorn and NoFap movements. Hundreds of thousands of secular men are fighting their battles with porn and self-abuse. Even science shows that the "rewiring" that porn does to the brain is undone when porn is no longer used. I also disagree that looking at pictures destructs pair-bonding, similarly to what fornication does to women. I think most any trad woman would prefer that her potential husband watched porn and self-abused thousands of times rather than fornicated with even one woman. I think men would agree in the opposite situation. Those priests are wrong on those points.
However, porn and self-abuse during a marriage can destroy it. Aside from taking away the man's desire for his wife, it can also cause him to pressure his wife into fetishes (uncomfortable in the least or physically/morally dangerous at worst) to satiate his porn-addled brain. It will cause the man to spend time away from work and his children. Not to mention the destruction of spiritual graces or even demonic activity that mortal sin brings.