The same thing that makes the viewing of pornography wrong for an unmarried person, also, makes it wrong for married couples.

(Matthew 5:27-30)

Pornography entices us down a path of sin, and at the end, quite often, it leads to the death of many things, such, as, a marriage, self respect, and a close walk with our Lord etc. (James 1:14-15)

The more one dwells on pornography, the more one becomes enslaved by it. We should not even take a second glance at pornography, for when we do, it becomes easier to take a third glance and so on.  Before we know it, we’re enslaved.  We’re not to let sin that has a way of taking hold of us and not letting go reign in our mortal bodies so that we obey its evil desires.

(Romans 6:12-14)

It’s not better for a wife to join her husband in viewing pornography in order to lend some sort of acceptability to it.  A husband caught up in this behavior needs to stop it; he needs to flee from it.