If there is one thing I've come to realize over the years with any religious people is that you can't explain their belief using their sacred texts. You say: The bible or Coran says this, and they'll say: Yes, but it also says that. And then you end up in an argument where the ultimate authority is not logic, but some thousand year old book that can be understood one way or another.
I find that relying purely on logic is often much better. Ask your wife what kind of relationship you have with her. If she finds plenty of positive things, then ask why that would not apply also to your son. If you are a happy couple, then you are the living proof, first hand account, that their rule is senseless.
If she can't find positive things and you find that you've been unhappy for several years, then, perhaps its time for you to make your own way. If she begs you not to leave, than ask her what's the point of saving a relationship if she doesn't cherish it.
If you do end up leaving, then, you'll have time to explain to your kids your views on relationships that is not dependant on the adherence to any religion.
I know, this sounds complicated, but it is not. It is hard to handle, very hard, perhaps devastating, but its not complicated.
When your wife questions the validity of relationships where the basis is identical to yours, its not complicated.