But in that case is a live webcam girl ok or a prostitute?
For me personally, there is a big difference: Using a live webcam girl, or a prostitute, involves a two-way interaction. This means there is some kind of one-to-one relationship between the two parties... and for me, that would not be acceptable.
Porn movies/images, on the other hand, are made to 'entertain' a mass audience. There is no level of actual personalisation involved at all.
The issue with porn for me is not about the relationship between the viewer and the actors (because there isn't one... at least not beyond the extent that I have a relationship with Paul Rudd, if I'm getting horny watching him take his shirt off on-screen!).
I do have an issue, however, with the misogyny generally that porn engenders, the way it (sometimes) exploits the actors and (often) reduces women to mere objects. It sucks.
I can imagine that, were I to have an issue with my partner watching it, it would not be because I consider him to be cheating; rather it would be because I'd think he isn't clever enough/is to lazy to consider the level of exploitation that goes on in the porn industry; that he hasn't made the 'right choice' and decided not to consume, and therefore perpetuate, this culture of misogyny.
...But then one starts getting into the murky issue of where one draws the line, regarding exploitation.... why is it not tolerable for him to watch porn, which exploits women sexually, yet is tolerable for him - and me - to continue to (for example) consume products which exploit people in other ways.
I mean, I buy my food from Tesco. They've been accused of exploiting their suppliers, tying them into contracts then squeezing their profit margins so tight these farmers are barely scraping a living. I make a bit of effort to buy direct from local suppliers, but I still shop at Tesco because I'm lazy. And I'm sure 99% of the people reading this do the same.
Similarly, I've bought Apple products. They have been accused of exploiting Chinese workers. But, y'know, I like using Apple products, so....
We all are (to some degree) culpable for some level of exploitation, somewhere.
Because we are all just flawed human beings.
That doesn't make the perpetuation of exploitation right, by any stretch. With more education about the realities of consumerism, and less inertia on the part of consumers everywhere, perhaps a positive difference could be made in many areas where exploitation is the means by which profit is made.
I'd encourage my partner to seek out the more non-exploitative, 'feminist' porn, but then I'd also have to be prepared to reciprocate and make more effort to consume non-exploitative products myself. And if I'm honest with myself, I'm not going to stop using Apple products, because y'know, I like them...