What is infidelity? Well, DUH! Everybody knows that. That's when a married person has sex with someone other than their spouse. Right? Well, most people would agree with that definition. But is that the only requirement?
Do you have to be married? If you and your partner aren't married, is it infidelity for you to have sex with someone else?
Let's say you're in a committed relationship with or without marriage. Would you commit infidelity if you had oral sex with with someone other than your partner?
How about if you just kissed someone else? Would it make any difference whether or not there was tongue involvement? Okay, what if you just hugged someone else? Even if there was no groping below the belt?
Would it be unfaithful of you to engage in innocent, casual flirting with a co-worker? What if this harmless flirting took place at a party with your spouse present? Would you answer the same way if it was your partner doing the flirting? What if both of you and everybody else at the party are flirting?
How about regularly having lunch with a co-worker and having intimate conversations about the problems you have in your marriage? No physical touching involved. Just "friends". Is that infidelity?
Imagine discovering that your spouse has been carrying on intimate conversations via email or IM with someone she or he has never met who lives in Tajikistan? Sexually explicit conversations. Maybe even with webcams. But they've never met IRL (that's "in real life" for the uninitiated) and it's virtually impossible that they ever will. Is that infidelity?
Are you being unfaithful to your spouse if you look at pornography? And what do you mean by "pornography"? What makes a photo or movie clip pornographic? If you fantasize about someone else while making love with your spouse are you being unfaithful?
Maybe you and your spouse decide to experiment with "swinging", "polyamory" or some other non-traditional relationship patterns. Is it infidelity if you both agree to having additional sex partners?
These questions aren't intended as some kind of pop-quiz to which I'll now give you the correct answers. Your religious values and personal ethics may determine what you think is the right answer for you, especially if you're just talking in theory. Have you and your spouse ever talked about these things even in theory? You might assume that your partner shares your views.
I can tell you there are growing numbers of couples who are discovering that many things that fall short of having genital-to-genital intercourse outside of the relationship feel exactly like infidelity to the partner who wasn't involved. Some who agreed to having additional sex partners besides their spouses find that it feels just like infidelity even though - since they both agreed to it - they didn't consider it infidelity. It hurts just as badly, and takes as much time and effort to heal.
If you and your partner haven't discussed your views in these areas, this might be a good time to do it. Especially if you're only speaking theoretically... so far.
So, what is infidelity? Ask your partner. You both need to know the answers.