The latter half is true, I think.

But I’d say there’s a significant chunk of folk* who will not hit on people they know are in a relationship. In the first month of your subscription this probably won’t have an effect, but by the sixth month...

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* This is obviously very cultural - who and where you are can change this a lot in either direction I imagine.

I think it actually depends upon the couple. People who care what other people think tend to date other people who care what other people think, while people who make up their own mind tend to date other people who make up their own mind. This trait seems to be more strongly assortative than pretty much anything else I've observed - intelligence, social class, beauty, etc.

So the grandparent's point is well-taken. You date folks for whom competition spurs attraction, because that's what you believe. Someone who doesn't isn't going to be attracted to someone who's always got a girl on his finger.