It's more likely that he is having performance anxiety and really is tired. I would be inclined to ignore the porn in his browser history. I mean, wow. Is there a guy left on the planet who has access to a computer who DOESN'T surf porn sometimes?

It seems like the issue isn't so much about him as it is about you needing reassurance and/or more sex. Both of those things have to be brought up directly with him, not with concerns about his lack of sex drive or computer porn, but with frank statements (not accusatory, just calm) something like: "I need the reassurance that sex provides. When we don't have sex, I start thinking something is wrong with the relationship. I'm not sure what to do."

When he answers, let him talk. If you can get him to talk, listen. It's probably not what you think it is. If I know anything about men at this point in my life (I'm 58), it's that it is hardly ever what we think it is. So try and get him to talk, but focus on your own insecurities and needs, not his. That usually goes better.

Good luck!