I'm not sure that the posts are explicitly only about sex... There have been a few (and this is me all arm-wavy and vague, because I don't recall the exact threads) where I wondered whether these people were just looking for a sexual threesome without anything else, or whether this was what (I think) most of us call "polyamory".
I think I made a mental note to ask for clarification (and to try to do it not in a snotty way) to find out what the motivations are (and are not). I don't want to push folks out - I think that a lot of folks start out by thinking that the only way to open up their relationship is with casual sex, and they may well have a lot more going on than just that, but the other stuff is difficult to express in typed words...
I appreciate everyone's feedback on these thoughts.
"Listen, or your tongue will make you deaf." - Native American Proverb