I'd qualify that to be "non-dating website".
But actually, I wouldn't be surprised if somebody broke off an existing relationship to pursue someone they were fantasizing about online. If they're unhappy in their current relationship, online fantasy is a form of escape and leads to eventual idealization. By comparison, the real person can't compete as their flaws are evident.
Dime-store psychology perhaps, but on these topics nothing surprises me.
I think many of the attitudes regarding porn has changed simply because it became so ubiquitous that it changed culture, not that culture somehow grew up and grew to embrace porn.
For some it might been seen as a light form of cheating.
I think that it's a sensitive enough topic that if you're doing it without your partner knowing that you're likely treading into a potential problem.
(For the record, I feel like searching your partner's computer for anything is a gross violation of trust - more so than watching porn without the other person knowing)