If we were looking for a third person in a "romantic" sense, it wouldn't be exclusively for sex. It would be a dating situation very similar to any other, only involving three people. If we were interested in you, potentially messaged OkCupid person, we'd just say that in any initial messages. If a message was sent to you, especially by me, that didn't contain any language of that nature, it's because that wasn't the thought behind the reason for your messaging; I probably just thought you seemed funny or otherwise neat.
While I don't want to have sex with every person I see/meet who is funny or otherwise neat, I have come to realize almost nobody on this website uses it for the "new friends" purpose, regardless of what's in their "Looking for:" section. That being the case, I thought I'd just do as the Romans do.
Regardless, I may still message people who are funny or otherwise neat for the purposes of reaching beyond my social circle for new characters.
Again, If you are on the receiving end of such a message, and I don't mention anywhere in said message that we would be interested in dating you, please don't automatically assume that I/we are being creepy. Trust me, the archetypical window through which we would both be able to find a person who was mutually agreeably attractive, cool, intelligent and otherwise appealing is so tiny that it might well not exist.
I roll my eyes as it dawns on me that I should probably clarify that if I/we do contact you, and that contact isn't indicative of any desire to date you, it doesn't mean that anyone thinks there's anything wrong with you. It just means we both want to be friends; just like any of your other friends.
If I/we look at your profile, but don't message/contact you, it doesn't mean anything.
Neither of us tend to message anyone until we've read their entire profile and have gone through at least 2/3rds of their questions (though, generally, all of them). That means I/we will generally have to look at a profile several times before contact is made.
I'm sure you're all wonderful people, or something.