It depends on what kind of relationship you have. Traditionally, and what seems right to most people is a relationship with a level of commitment towards the other. I'm not saying this is always true, but that level of commitment does have benefits, usually resulting in a stronger relationship because of the trust developed.
Associating watching porn with lack of commitment sounds like something out of the 50s. Nobody(*) contemplates leaving their SO for some person on a website (or in a movie, or on TV), and porn isn't an exception.
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.
Probably closer to more like something out of the 90s. Also very much age / culture / internet age related.
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.
