(I agree that there are people who consider looking at porn or reading stories or even maasturbating to your own head-movie cheating, and this is in my opinion completely ridiculous, and I would not put up with such a person for long.)
I agree that there are things your spouse cannot provide you with, for whatever the reason may be, but I do not regard that fact as proof of "wasting your time with the wrong person".
Expecting ONE person to fulfill ALL your needs is a huge task. I consider it unfair to demand that from one single person.
I have one online person I share my non-possible fantasies with (quite splattery, leaving permanent damage or me as a corpse behind if taken to real life). Hubby knows this, and the best parts I send him to read.
He also has most of my passwords etc. and COULD basically sneak into my activities, but he says he does not do that (and I believe he would only do so if he was seriously worried about anything going on, or if he had the feeling I withdraw from him and give myself to someone else...).
He did not ask me for them, and I would not stay with a person who "demanded" it from me. I want a partner, not a fucking parent or control freak.
On the other hand - I gave them to him because he did NOT demand it, and therefore it was easy for me to do it, I did not feel mistrusted or pressured.
The one thing he wonīt have is me doing anything physical/sexual behind his back online (I do not want anyone else offline/in real life, mind - however, if hubby left me for someone else, I might very strongly consider a visit to my online playmate *g*).
I, on the other hand, do not want his passwords and a full report of his online activities. I even believed for more than 10 years that he did online chatting, because he knows all the pages and stuff that is BDSM-related :-)
I never asked him, because I donīt mind if I donīt know it, as long as I donīt feel anything vital is being taken away from me/he rather spends his time online with others, than with me.
So "doing it behind your partnerīs back" is ok with me, BUT! that is because I would want it that way, and not because we never spoke about that and I took his being "good" for granted.