No, I don't think it's normal. I'm not sure if it's unhealthy though. Abnormal doesn't mean unhealthy: homosexuality's abnormal after all and I don't know anything directly unhealthy about that.
Now, if you were a sex addict, yes because as I understand it the word addiction implicitly describes an unhealthy fixation on something. In this case, I guess whatever neurotransmittor release that occurs in regards to various stimuli.
You could explore your motivations, like why you enjoy certain things. In some cases, a man may just really enjoy seeing the one he loves enjoying herself, and if he doesn't get that response himself, seeks out assistance.
On the other hand, if your wife was more pleased with you sexually than with other men, then one wonders what the appeal would be... could we have reservations about direct sexuality where we enjoy it further from a distance because it has fewer pressures?
Or perhaps there is a greater amount of honesty with a woman's responses: if she has sex with us, would she fake an orgasm, but if it's with a stranger then perhaps we think she would give more honest feedback?