Weigh in if you want.
This is regarding people who are supposed to be in relationships already. If you're single, then have at 'er. I couldn't care less.
but...
I think people have secret on-line relationships because they are selfish uncaring bastards (or bitches). Sorry, I don't care what kind of crap is going on in their lives. If they have to keep it a secret then they know darn well they are doing something they shouldn't. It's the most cowardly way of running away into escapism land. Finding some mystery person to talk to, who will understand you rather than working on understanding with your real life partner is infinitely easier. You can pretend you have no flaws and that this person doesn't either. Yay! Good for you! Congratulations at achieving happiness at your partner's expense.
If you have problems with your partner, isn't better to somehow communicate that issue rather than sweep it under a rug?
Are some of these people honestly so naive that they think their little on-line romance will just stay on-line? If you are looking for romance outside your partnership then you are basically looking to have sex outside it.