This is a tough situation.
Women who have endo are often in a lot of pain. The pain alone can cause a lack of desire for intimacy, but there are also hormonal issues involved.
Is it fair? No. But does it happen? Absolutely. You have to make the decision of whether or not you want to stay in the marriage under those circumstances.
But sex is not the only type of intimacy. There are other things that you two could do that would not be painful. Have you suggested them?