Your life really touched me. I don't know what to say.
I heard that some men don't get excited by their own wives, but by different women. They look outside (and sometimes by paying for it) what they could find at home and in much better quality. I guess if it's a kind of illness.
I think that if you wanted to save your marriage, you should go directly to him and look for a marriage therapy. I think he should have any problem.
Maybe, at beginning, he was shy to ask you for "specific" things and he started looking for it outside, and now he just cannot stop. I don't want to excuse him, because he is not a boy with a chocolate and he should control his body and his needs, but i am just trying to look for an answer.
I think the only way out of this situation, before you fall in something you obviously don't want to fall, is to go for therapy as soon as possible.
The answer may be to go ahead with a healthy marriage, or to finish everything and stop the nightmare. But believe me, even if he was a good, the kind of relation between the parents will affect sooner or later to the children. I saw it thousand times, marriages that keep together just for the children, and they finish hatting each other. It's the worst.
Try to fix it for last time, and try your best. Try to make a research on where is his problem, because i am sure there is a problem if he was doing from the beginning of your marriage.
Dear, big hugs and focus in your children in these moments, because they are the best of God's gifts.
No God wants the killing, but the peace.
The weapons are carried by people, not by religions.