Being a Christian, I am against the whole porn thing and the cheating thing. Of course there was a time in my life when I took part in both. I am not proud of it but it happened and I regret it but I cannot change it.
The lack of sex could be that he is embarassed about his body and is afraid that seeing him naked or being with him while he is naked and is afraid that it will turn you off. I know, my wife is like that. She is a large woman and we will have been married for five years this December and she has never let me see her naked. It doesn't matter how much I tell her that I do not care about her size and it doesn't matter how much I tell her I love her, she is very uncomfortable about her body.
Maybe you could rent a porn movie or something, get naked or dress in some sexy lingerie and ask him if he wants to watch it with you and then see if you "can get him to rise to the occasion". If that works then some night if you guys are ever just sitting there watching television together, just do things that may get him aroused and see what happens.
I agree with everyone else that divorcing him would be less hurtful than sneaking out behind his back and having an affair and then him possibly finding out about it.
Maybe you two could go see a sex therapist or something too.