Well, sex is a huge reason why relationships fall apart. Men see sex as love and if it is getting less and less frequent with their wife/girlfriend then in their minds they will think "she does not love me anymore". Also, what happened in the relationship prior to this one? Often men and women will have scars to their mindsets from previous relationships. If your previous boyfriend/girlfriend cheated on you then perhaps you might feel the next one will do the same, and so you feel less secure in the relationship and risk it on another.
And without sex, sexual frustration kicks in! Men (especially) will suffer bouts of depression if they are forced to go without sex because their wives are not feeling it anymore. And with this depression they will look to other things to interest them, whether that be a PlayStation 3 or another woman.. And like I said before, if they feel unloved, well
I think both people need to make an effort in marriage, try to meet the others needs whenever possible. Any relationship can be a struggle at times, but only if both parties understand that and understand each other, can they get through those rocky times together.