What's up with my husband's sex drive?

I met my husband online and we had a long distance relationship for 5 years before I moved to be with him. One of the reasons I wanted him was due to his strong sex drive, I wanted a man who could match mine. He assured me all the time that he was active in that department and with all the phone sex and whatnot, I had no reason not to believe him. We had a few meetings and everything seemed great. Then he asks me to move in with him and marry him, and his sex drive disappears. We were only having sex twice a month. He said he just needed to ease back into it as our sex was occasional for a few years. 4 years later and its still the same. I found out that he was looking at porn after I went to sleep and before I woke up in the morning. I also found that he had signed up for an internet dating site while I was living in another city. The profile says he's just looking for sexual partners to help him with his "overpowering sex drive." What happened to it? Is it normal for it to just disappear like that?