There are a couple of things here.
I guess it depends on your perspective but viewing pornography and phoning chat lines is, for most a very seperate issue to cheating and infidelity.

One is done to the exclusion of your partner, the other can often be done with your partner in mind. Admittedly not always. But most men and some women go through phases of curiosity or desire to experience something new and pornography is an easily accessible, and largely accepted way of exploring your sexuality.
What he definitely isn't is some deviant or borderline sex offender.
(My partner is a RSO, I am familiar with the behaviour.I have also done alot of work with online SO's.)

What it does sound like is that your partners has particular fantasies that he hasn't felt able to share with you, as he is perhaps ashamed of his desires.
Would you be able to talk these through with him or is this a topic you yourself find difficult to discuss.?
Is it possible you could both see a relationship or sex therapist together to discuss this.?

I hope you are able to work things through and wish you all the best for the future.