Porn like any addictive activity / substance is being used to obtain something they are not receiving emotionally, past or present..... the persons drug of choice.
And while it maybe natural for a man to be visually drawn to the site of a beautiful female - it is not natural (no matter what they say) for them to use them in place of their sufficient other.

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Now with that being said and from my own experience in this area of a wife whose husband was once addicted to porn for ten long hurtful years (6 years strong now without it) - I have to say that your husband is probably using porn to relieve some of the tension he is feeling from the hard work he does and from the lack of intimacy (not sex) in the marriage.

So.... What can the two of you do to strengthen the marriage and to grow closer to each other and not farther apart?

P.S.
Many Happily married Men & Women use Porn..... for it is not about the spouse but more about the person using it, therefore, the spouse can love you (with out change) and use porn all within the same day.