"The only moral principal I have articulated is that I do not think that people should do things which clearly on net add misery to the world. Let's be super clear about this: are you saying that's a bad moral principal?"
You do whatever you like, but it's worth noting that not everybody's definition of net misery is the same as yours, and to many, discreet encounters run the gamut anywhere from morally fine to morally evil, no matter how far adrift your own moral compass is from the median.
There are people using the site in ways for which I'm sure you'd approve, just as there are undoubtedly people using the site in ways for which I'm sure you wouldn't. That said, that's all irrelevant, as my only point is that getting people to agree with you isn't the high water mark for correctness. Otherwise smart people routinely agree on really dumb things, and referencing my earlier post, I'm sure that there are plenty of people who disapprove of gay marriage who would be able to put their views into similarly noble phrasings.
Also, I did not assert that everyone should be in an open marriage, or involved in polygamy. I don't care what kind of marriage you're in, or anybody else. I just don't think that "society says so" is a valid reason for allowing or disallowing other people from getting married. That same logic was being used to discriminate against interracial and gay marriages, and is currently being used to discriminate against polygamy and polyamorous marriages. None of those marriages take anything away from mine, so whatever I might personally think of people I cannot identify with, their right to marriage is no business of mine.