Well, look at it this way: Is there any similarity between “having an actual affair” and having sex with a prostitute while you’re married? I think most people would answer yes. Then consider: Is there any similarity between having sex with a prostitute while you’re married and paying to watch a prostitute perform sexual acts for your voyeuristic gratification? Again, I think a lot of people would say yes: There’s a distinction, obviously, but I don’t think all that many spouses would be inclined to forgive their husbands (or wives) if they explained that they only liked to watch the prostitute they’d hired. And hard-core porn, in turn, is nothing more than an indirect way of paying someone to fulfill the same sort of voyeuristic fantasies: It’s prostitution in all but name, filtered through middlemen, magazine editors, and high-speed internet connections. Is it as grave a betrayal as cheating on your spouse with a co-worker? Not at all. But is it on a moral continuum with adultery? I don’t think it’s insane to say yes.
I think Jon Chait pretty much nails it here. By parallel reasoning, of course, any kind of fantasizing is on the same “moral continuum,” with or without visual aids. But this is all pretty casuistic: We move from case to case without direct consideration of what the objectionable features of adultery are. This gets a little tricky because, of course, what counts as adultery is a function of the understanding, explicit or implicit, a particular couple has. Some couples, after all, go in for “voyeuristic gratification” together. So what we’re really talking about is what we think a reasonable modal implicit contract is about. One obvious reason adultery is typically ruled out is the risk of contracting a disease from or impregnating (or becoming pregnant by) another partner, which obviously isn’t an issue here.
The more relevant problem is what we might call, loosely, betrayal of exclusive initimacy. But this is where mediation makes all the difference. You don’t have a “relationship” with Aurora Snow by dint of watching one of her movies; you’re certainly not at any great risk of running off with her. All of which is to say, porn is not really a substitute for the sort of gratification that comes from real intimacy with a partner. I’m going to suppose that nobody’s implicit understanding involves an agreement not to get “gratification” from any other source—a nice glass of wine, a good novel, a game of pickup basketball. Obviously, porn is not quite so qualitatively different as these things, and I can imagine a particularly wretched sort of relationship in which it really did serve as a kind of substitute, rather than a complement. But it seems unlikely that this would be the case with very great frequency.
Addendum: I should probably stress a little more how much goalpost shifting is going on here. Think of it this way: There’s no good reason, once we’re in crimes-of-the-mind territory, to stop with hardcore porn. Ogling a scantily-clad Angelina Jolie in a mainstream film, after all, is a way of getting a certain species of sexual “gratification” from someone other than your partner, whether or not there’s an orgasm involved. And indeed, someone might well get upset if their partner seemed to be doing this a great deal. (Though I suspect less upset, in most cases, than if the same partner was ogling a real live person on the street.) But the question isn’t whether some people might have some reason to feel upset about this. The question is whether it makes sense to talk about anything in this broad sphere that might be upsetting to a partner as basically the same as adultery. Perhaps Ross will find it more persuasive if I put it this way: If we don’t stand strong in defending the traditional definition of adultery as the union of one man and one woman, we risk rendering the institution meaningless. After all, if adultery can mean anything, then it ultimately means nothing.