When sexually cheating on a partner is a repeated behavior, I have sympathy only for the one cheated on, as I see the cheater as a liar. But the one-time-slip-up kind of cheating? Part of me finds it horrible and destructive and selfish, but a large part of me feels intense sympathy for the cheater. Maybe this is because I've had so many dreams about 'accidentally' cheating on Ben, because (in dream reality) I forgot I was with him. I know this isn't logical in real life, but my subconscious says that there is a kind of insanity, a kind of drug-rush, that can happen in almost the same way. Like a person dying of thirst might kill someone for water, no matter how much of a pacifist they might be. Don't get me wrong -- I still think it is a horrible thing to do, and not permissible or excusable in any way -- I just... feel for both parties. Especially when the cheater genuinely loved the person they cheated on.
I used to be absolutely petrified that I would accidentally cheat on Ben, or that someone might mistake me for having cheated. I could just see how that would crush him -- how I would have broken every line of trust, made every act of love seem like a lie. And I didn't even have a desire to cheat! For half the time I was having the dreams of 'accidentally' cheating, I didn't even have much of a sex drive, and certainly didn't know anyone nearby that I would even want to have sex with. I think that my subconscious was trying to tell me that I was not meant to be monogamous, and that fact was a time bomb in the structure of my relationship. Not that I would be 'forced' to cheat or any such ridiculous thing, but that if I fell in love with another person, it would have the potential to be destructive to my relationship rather than beautifully creative -- because the structure of my relationship made it so.
Some people have the mistaken concept that polyamory means cheating when nobody is allowed to get mad about it. Not so! Cheating breaks a polyamorous relationship just as it breaks a monogamous one. Cheating is promising a person something and then not giving it; in monogamy, it means telling a person that you won't share sex with anyone else, and then doing it anyway and turning your promise into a lie. Cheating hurts because it breaks trust, because it is the biggest lie you can tell. Cheating is breaking the rules, whichever those are. Monogamy and polyamory have different rules, and make different promises. It is just as possible to cheat in a polyamorous relationship! For instance, Ben and I have the promise that we will get the other person's approval before embarking on any romantic relationship, because we trust each other to have the wisdom and generosity to help us choose the right people. If we were to have an additional romantic relationship WITHOUT getting approval from the other, that would be breaking the rules of our relationship and cheating.
Some believe that cheating hurts because an individual only has 100 points of love they can give at a time, and if they give to two people then each person only gets 50 instead of 100 -- less than they deserve, less than they can live on. But love doesn't work like that! It's not measurable or divisible. When you give love back and forth it increases on both sides, it doesn't stay the same amount -- that's the magic of it. And your heart doesn't have role-slots to be filled by specific people -- you can love more than one person with your WHOLE heart. Sometimes one person might be drained or hurting and not have a lot of love to give, but that is true of a monogamous or polyamorous relationship, with one difference: in a polyamorous relationship, the hurting one has more than one lover to pour into them and help them get back on their feet. A person does have limits, but some people have a limit of several partners rather than one.
Because this post is kinda all over the place, have some links to help clarify: a site that answers common questions about / reactions to polyamory & why I am polyamorous.