Read Everthing on this site. Everyone here has been through something very similiar if not identical to what you have. There is a wealth of information and suggestions here from so many people. Others will also. However, as you state his access to the computers is somewhat limited which would mean that at best this is may be more a curiosity thing.
Men do find sex interesting and pictures of women interesting.
If you want honesty in your relationship, pick you battles carefully. The porn sites is not an issue to break up a marriage over if it is a curiosity type of things, although with your stated views you may think so. Further, if there is an internet affair going don't confuse issues. Address that issue. Finally, if you want the honest answers to questions be ready to accept the answers and not blow up.