A: This is a situation I see more and more often as the Internet becomes a staple in homes.
Two main issues are in play here: honesty and uncommunicated expectations. First, your husband offers an explanation for his behavior that is possible, but not highly believable. Unless he informed you or gave you the chance to participate in these activities, it was not an honest act on his part. He also brushed off a chance to set the record straight without lying.
Second, there's the issue of expectations you two have not discussed. This situation underlines the importance of checking in with one's spouse or partner about the various aspects of long-term relationships: finances, in-laws, jobs, children (if any), housekeeping, plans, and sex.
People assume that because they are with a person they know pretty well, they also know how their mate feels about these topics. Often they are wrong. This is what keeps marriage counselors busy.
So, even though you said your vows and have been together for a while, it sounds like the topic of online chatting and surfing porn sites never came up.
The Next Step
Sit down with your husband to have this discussion. If he has sexual desires that he talks about with women online, then ask him to tell you what those are. That way you have the opportunity to say, "Yes, I'll do that," or, "No, I won't." You can also ask what prompted him to think that he was doing right by you in this marriage by finding sexual satisfaction online.
Evaluate yourself as well: Have you been shutting him out sexually? Are you enjoying yourself when you have sex? Do you have fantasies of your own? Is it a time of connection between you? Are there risks that you might be willing to take in terms of new behavior?