For that matter, anyone, absolutely anyone, is also completely within their rights to not be interested in sleeping with someone who's married even if it IS a 100% consensual and informed open marriage. Even poly people. Before I met my old man on the side, I was dead set against being anyone's "second".
Partly because of having dated several men in "poly" relationships who turned out to actually be in the process of divorces brought on by their adventures in open-marriage land, ie. their wives found someone they liked better. Dating people who are in the midst of marital turmoil sucks ass. But that's an aside.
Oh, I wouldn't touch the situation with a ten foot pole. But "I'm not interested in dating someone who's spouse doesn't know," is a fully adequate statement of that. Throwing in "I'm not here to help you cheat," doesn't accomplish anything but saying "and this is how I judge that behavior."
I'm not saying that's right or wrong. I'm just saying that it is judgmental, assumes something that may or may not be true, and there's a possibility that it's being read by somebody who it's unduly dickish towards.
If I threw up a profile that said, "I'm not interested in being a part of a poly/open situation. I'm not here to accommodate deviance." I would imagine I'd get a message or two like the one this dude sent.
Oh, fuck that. "I'm not here to help you cheat" is direct and to the point. The cheater knows he's cheating, and anyone who isn't cheating knows it's not directed at them. Judgmental? Sure. It's meant to send a loud and clear message to THAT PERSON, WHO IS IN FACT CHEATING ON THEIR SPOUSE, that she's not fucking interested. Who gives a fuck if it gives them a pang of guilt because their conscience isn't clear?
It doesn't say "I'm not interested in helping you cheat, you lying scumsack". Should she use a friendly euphemism for cheating? It is what it is.
Are you saying that merely CALLING it cheating, when it is in fact cheating, is somehow harsh? I think the argument could be made for A. No fucks given, and B. If someone is on OK Cupid and looking to cheat, if they are so fragile that seeing it called "cheating" is going to injure their ego, they need to get the fuck off OK Cupid because they are in no way up for the emotional rigors of dating anyone. Period.
Basically, fuck that guy, he can go fuck himself. If he doesn't have the emotional fortitude and intellectual honesty to simply SEE the word for what he's doing on the profile of someone who isn't even interested, he can't handle doing it.