It is very very true that if someone really truly wants porn, he (by far most consumers are men) will get it somehow, if only by dreaming it up himself. However, keeping it from being too convenient and easy to get is a legitimate social goal. Bans, even if they are only limited local ones, have ripple effects. They do play a part in shaping social behavior through pride and shame (gak! what totally Neanderthal concepts).
If porn is demolishing a marriage, there is usually a lot more going on than some naked pics. The only women I have known who have had problems with porn are the frigid cow variety that think that the ring on their finger means their husbands are going to be happy with vanilla sex doled out once a month. Their problem isn't porn, but usually a much deeper breakdown in communication.