Her anger was clearly out of proportion to reality. We didn't do anything wrong, and her reaction was a red flag that she doesn't trust him. To her, even his minimal contact with an ex is problematic. Former lovers often cause jealousy to erupt, because their claim is so intimate and personal; they knew the person you consider uniquely yours in a way that feels threatening. Yet her behavior is likely to push him away with its intensity.

Almost every relationship ultimately demands that we ask the question: What is cheating? The frustrating truth is that there can never be an objective standard, because everyone defines it differently. A guy started flirting with me, sending innuendo-laden notes, but when I invited him over, he started backpedaling. He told me he was in love with someone who wasn't kinky, but he wanted to have a kinky fling with me. I told him he'd have to decide for himself. He chose me, and though we didn't technically have sex, I'm sure his girlfriend would be appalled to find out that he let me spank him while he writhed across my lap like a baby.

I'm reminded of the time my best friend, my ex-girlfriend, and I went to see the raucously sexy musical group Pink Steel. After the show my ladies took over the stage and dirty-danced, their bodies grinding together in an over-the-top way. While I sat and watched, a friend came up and asked, "Doesn't that make you jealous?"

I said no, because I knew they were enjoying themselves, and there was no chance of anything happening beyond their onstage shimmying. The line between friendship and flirtation is often so minimal that it might as well not even exist. Having watched these two spend plenty of time together, I couldn't suddenly cut in and say, "I'm sorry, please keep the boundaries of your friendship neat and even."

Almost every relationship rule exists to prevent jealousy. Yet the dirty little secret is that, like all emotions, jealousy doesn't listen to reason. What our rational minds know to be a partner's harmless flirtation often feels like an arrow aimed straight at our hearts. We might be jealous of exes, co-workers, or friends of our beloved; it's not necessarily fair, but the feelings are real, and partners need to take those emotions into account. For me, when jealousy strikes, it feels like a monster taking over, making me turn colors and become enraged.

I don't like being the person glaring at a girl across the room and muttering things like, "I'm hotter than she is, right?" as I recently found myself doing. After watching a stranger at a party grope the guy I've lusted after for six months, I downed a martini—my first drink in over a year. Seeing her cuddle up to him made me instantly hate her with a ferocity that scared me, and I couldn't handle those feelings sober. The good news is, succumbing to jealousy can propel us forward; rather than slinking away to cry in the bathroom, I expressed my hurt in whispered invective and petty gossip. But being suspicious or hovering over someone makes us seem less attractive, and is likely to turn a fear about cheating into a self-fulfilling prophecy.

Just because someone's vowed to be faithful doesn't mean that vow is being upheld. Many married men like to flirt—boldly, publicly, and provocatively—as if to say, "I can chat you up and turn you on, maybe even fuck you, then go home to my wife." From the safety of their relationships, guys like these get to have their cake and eat pussy too. Their wedding rings are supposed to signal to potential mistresses that they're only available for sex. Sadly, many people see this as business as usual. Cheating husbands are such a cliché I feel silly even mentioning them. When I witness such behavior, I always wonder if their wives know what they're up to and simply don't care.

My friend Jenny recently announced that she and her boyfriend of six years, who've been alternately polyamorous and monogamous, just decided making out with other people would be OK. I confided that sometimes when I'm dating someone, it turns me on to watch him or her kiss another person, and she squealed in agreement. "Most girls don't think that way," she lamented. We're conditioned to see our partner's attraction to someone else as an automatic signal that they don't want us anymore, which often isn't the case. Desiring others is natural; I'd find it odd to date someone who never visualized getting busy with another woman.

I'm more concerned about emotional fidelity than physical, but even that maxim utterly depends on the person. I feel incredibly possessive about certain people, and when I see them with someone else, I can't handle it. Even if I technically have no claim on them, their public displays of affection still hurt, and that pain is something no amount of alcohol or logic can cure.

I used to be extremely high-minded when it came to cheating; it was wrong, all the time, under any circumstances. Having been on both sides of the situation, I've learned it's much more complicated. I believe honesty is more valuable than strict physical monogamy. I would rather have lovers share their complex fantasy lives with me than be technically faithful yet thinking about someone else when we have sex, another questionable instance that surely isn't "cheating" but may feel even worse.

We've probably all experienced intense jealousy (and if you haven't, I truly envy you), but each of our breaking points is different. For some it's a phone call, an e-mail, a flirtatious glance; for others, it's kissing, sex, spending the night, or a declaration of love. The tricky part is knowing what triggers our jealousy and learning to manage it so we can deal with people honorably, without letting the green-eyed monster dictate our behavior. In that department, I have my work cut out for me just as much as the next person.

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