Well, I've avoided this issue long enough. In all my posts on adultery and the responses to reader comments, I've never answered the question: what counts as adultery? Sexual intercourse seems obvious, but what if it was "meaningless"? Or what if there was sexual contact but not intercourse? Or maybe "just" kissing—is that better than "just" sex, or worse? And what about love without sex? And what about very close friendships, such as one commenter in the last post has with a woman in a committed relationship?
Maybe I should keep avoiding the question, after all—hey, what's up with Lost, anyway? That's just messed up.
Uh ... anybody got a good bonobo story? (Just kidding, Vanessa.)
The easy answer is that there is no answer, if by "answer" we expect one standard that will satisfy everybody. Some people are more upset by sexual activity, others by emotional entanglements—the latter would presumably be more upset with a kiss or a tender email than a pair of unfamiliar underwear under the bed. Some couples agree to open sexual liaisons outside the relationship, but also agree not to develop feelings for their other partners. And some adhere to a rule of "anything but sex."
Is there a pattern here? All of the examples above are stated in terms of what the other partner will or will not tolerate, what makes him or her upset, or what couples have agreed to. In words, adultery is in the eyes of the one being cheated on. (That sounded snappier in my head, honest. Really. Hey look, there's a bonobo!)
It stands to reason that if one of the main wrongful aspects of adultery is betrayal, then whatever represents betrayal to a particular person is what counts as adultery on the part of his or her partner. If sexual intercourse makes a person feel the most betrayed, then it counts as adultery to him or her. And if kissing makes another person feel the most betrayed ... you get the point. (Of course, deception is also an important component of adultery, but the deception is in reference to some presumably adulterous behavior, so we're back to asking what behavior counts as adultery.)
It's no use saying that kissing (or sex, or romantic emails) is adultery if not every person would be upset by his or her partner kissing (or having sex with, or emailing "<3 <3 <3" to) somebody else. But if betrayal is an essential part of adultery, then whatever betrayal is to a particular person, adultery is also. That also explains the view that something is adultery if you wouldn't tell your partner about it—because the issue is what your partner would be upset with, and different people get upset about different things.
But what if you disagree with your partner about something you're doing? Suppose that you're enjoying romantic dinners with a friend from work and your partner accuses you of emotional infidelity, but you don't think he or she should be upset. (First of all, do not tell your partner that he or she "should not be upset"—even bonobos know not to do that.) You don't feel like you're doing anything wrong, but your partner does. So who's right?
It doesn't matter who's right or wrong; what matters is that you disagree, and that is something that you have to talk about. Your respective views regarding boundaries are part of your values, and part of who each of you is—neither of you should deny how you feel, but if you can't reconcile your beliefs about something as emotional, personal, and visceral as adultery, then this may be a problem in the relationship, and better you deal with it now before much (or more) pain results from it.
So what counts or doesn't count as adultery is an issue for each couple to decide for themselves, depending on what boundaries they're comfortable setting—as well as how forgiving or flexible they'll be with them. Ultimately, it's not the act that's wrong, but how it makes your partner feel.
You can follow me here if you'd like (now with 25% more bonobos in every tweet).