I'm going to agree with Tom on this one. Unless a partner has explicitly told you that it is okay to have dalliances and that they don't want to know (as in the case of Ozme), it is extremely disrespectful to assume it is okay. It is particularly disrespectful to assume they wouldn't want to know because you think they'd be hurt if they knew. I'd rather be hurt and know than be happy without knowing. My happiness would be based on a falsehood and misplaced trust. To me, that's the ultimate betrayal. Not only that, but if you assume they wouldn't want to know, it means you think you know what's best for them. That is a very patronizing attitude, and definitely out of place in a vanilla relationship.

An online relationship is different from porn because of the word "relationship". Now, if you're talking about supposed one-night stands or something like the Academy, it gets a little trickier since, unlike real one-night stands, you can forget about the other person and pretend they aren't real and thus, just a stand-in (which makes it more like porn)... but even then, I think you should be able to let your partner know, and if you don't, you need to ask why. This actually goes for porn, too, though. If you're secretive about your porn and not willing to say you're doing it, there's probably a reason, and you should ask why.

The answer to that why will probably lead you to realize that your relationship is lacking in trust. And that's not something that will be fixed through anything external to the relationship: it has to be fixed, if it *can* be fixed, within the relationship. If it cannot be fixed within the relationship, then you have to decide whether you can accept a relationship without trust. But trying to have both is disrespectful both to yourself and to all the other people involved.