Not to long after hitting puberty, people start to learn about forming and being in relationships. (This being the society it is, this happens through experimentation rather than teaching, but that's a side-issue.) Of course, this doesn't happen to everyone -- some people miss out on this because they're unattractive, or socially inept, or simply don't have the opportunities. This "learning phase" continues for a few more years, to the late teens (this point seems to be highly influenced by social circumstance).
At this point, a "doing it for real" phase is entered. People apply the skills they've been learning over the past few years to serious relationships of varying degrees of permanence. It's quite obvious that those who didn't go through a learning phase are at a severe disadvantage -- they're either going to have to find others in a similar position they can work on a late learning phase with (this ties in to the bisexual geeks theory) or not get anywhere at all. Again, this continues for a number of years, by which time people will have fallen into, roughly, one of three kinds or relationship: committed monogamous, multiple (serial or parallel) or none.
The question now is, how are the people in the second category (and the first, if/when they break up) going to regard those in the third when they're out looking for someone? What they are going to see is someone with no track record, which implies that they have previously been regarded as unattractive or socially inept, or that they lacked opportunity. The former two are obviously immediate turn-offs -- they might have improved, but it's a bit of an outside chance. The latter means that they are still at the learning stage, and thus comparatively socially inept -- while they may be able to learn, it's a tedious process for an experienced person to go through. Particularly when there are enough people available from the first and second categories who are known (or at least strongly believed) to know the ropes.
The upshot is that the people who have few difficulties in finding new partners from their mid-twenties onwards will be observed to be those who have either never had any difficulties finding new partners, or who have been in (a) solid long-term relationship(s), while those who don't establish such a pattern by their mid-twenties are likely to find themselves stuck in an ever-deepening rut of solitariness.