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)
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.