I'm specifically talking about married people meeting people of the opposite sex on-line, and that it's bad and wrong.
I think it means they didn't marry well. That happens, a lot.
I don't agree their hearts were necessarily gone before they turned on the computer. The computer could have been a legitimate tool for some legitimate purpose. However, without recognizing the temptation it can present, things can quickly become confusing.
It is the same with anything to which one chooses to give their time, but I think the internet holds an unrealized dimensional quality. And if there is already the suspicion felt(even, perhaps, not consciously recognized) that there is a lack in their relationship, the reality of what is truly pulling them to these outside interests could be honestly lost on them.
As for whether it proves they married well, isn't it true that even "good marriages" have ups and downs, conflictual periods that, if handled wrong can make it "appear" to have been a "poor marriage" from the start?
There seems to be another point of view offered here by some that to even "look" is bad. Now I can agree that finding that there appears to be something better out there can often be counter-productive to happiness -- for all the reasons we are aware of.
But I just can't pronounce that bad or evil per se. Everyone has to make that call for themselves.