It is to some people - I know I'd be bothered by it. Mostly due to my own insecurities. I'd wonder "Why?", and wonder what I'm not doing that he wants.

For some it might been seen as a light form of cheating.

I think that it's a sensitive enough topic that if you're doing it without your partner knowing that you're likely treading into a potential problem.

(For the record, I feel like searching your partner's computer for anything is a gross violation of trust - more so than watching porn without the other person knowing)