I think there are so many factors involved. It depends on the connection/relationship you have with your partner (some people are together, and ok with an open relationship), the level of commitment, your sexual behaviour and standards, and the degree with which you connect love and sex.
Some people don't connect the two at all, making it easy to cheat, or easy to be sexually attracted to others and let their SO know. Emotionless sex isn't impossible, but its not what humans strive for so much. Humans don't want just sex, they want great sex, and lots of great sex.
I'm sure most will agree that connective/emotional sex is much better than a quick fuck.
Now for the question
My boyfriend and I are committed to each other, and were each other's first sexual partner. We are friends first, and lovers second, so we have a 100% honesty pact. No bullshit, no keeping things from each other.
I asked him once how he figured he would act if he cheated on me. No bullshit, he'd let me know right away. I respect that.
Now, if he asked me before-hand. If he was sexually attracted to someone else and asked me if he could fuck her, well, of course I'd be upset. Why? Its not the lying or the dishonesty because it doesn't exist in this situation... he's telling me before the cheating what his desires are.
I think its simply the insecurity, and the "what-if?" factor. If I was in a situation like that with my boyfriend, I think I would immediately wonder why he feels he needs to fuck someone else with no emotional connection to, and what if one develops between them because of it? Am I that easily replaced? And if I am, does commitment really exist? Are humans capable of it for Christ's sake?!
That fear of being replaced is what immediately puts that terrible feeling in my chest. If we can be so easily replaced, no relationship is meaningful, and suddenly, commitment means nothing.
Lust can be a powerful drive, but it does not control us. To give into that lust, when you have something with someone, and cherish that connection, makes no sense to me. Why trade lust with love? Lust is always available, love isn't. REAL love, that is.
Now, my answer is based on the level of commitment. Plenty of couples out there aren't fully committed to each other. If you don't feel really connected to that person, or if it's not mutual, cheating is a lot easier to do, for both parties. But if you're in it for the long run (aka marriage), well, it'll definitely be harder. heh. Or maybe, you're just denying your real level of commitment to your partner, and to yourself.