Referring to the past history of your husband's affairs and his addiction/weakness towards porn watching; there could be couple of reasons for this. Before going through the process of assessments and probing into the causes, please do talk to your husband openly and express how you are feeling about the entire thing, how insecured you feel about the relationship etc.  Ask yourself if your bonding with him is strong enough, there are many other things apart from sexual relationship between a couple that plays vital roles in the relationship.  Mutual expectations, understanding etc.  If those are in place you will be in a better position to tackle him with this issue.  There are probabilities that he is addicted to porn and using you as an instrument. In long run, there are other serious issues that would pop up. Both of you consulting a Family Counsellor will be suggested.

Hi, I can understand your concerns considering your husband's past history with affairs. In such a situation, open communication with your husband is your best option. You can try explaining your concerns to him through an email or a letter. See how he responds. Once he understands your position, he might be able to explain his behaviour to you. If nothing works and it affects your relationship with him, do consider involving a neutral third person to open the channel of communication between you two. You can also try going to a marriage counselor. Also, you can speak to your girlfriends who are married and try to understand their opinion about their husbands indulging in porn. You can read online about sexuality and different ways used by men and women to express it. Hope you find a way to resolve this issue with your husband soon. Good luck!