I think people might underestimate the religious side to this. I'm Dutch, and the vast majority of Dutch Evangelical pastors, elders, or youth leaders avoided meeting church members in a 1-on-1 situation across gender.
There are some obvious reasons for this, like false accusations by the many less-than-stable members, as well as the therapeutic nature of many meetings (rife with possible line-crossing from both sides, psychologically).
Less obvious reasons would include a more fundamental belief in the 'sinful' nature of the flesh. Again, the vast majority of Evangelical leaders I know strongly believe that even the best of people can be tempted into sinful acts, and a pragmatic solution is to avoid the temptation in the first place.
I might be wrong, but I get the impression that society in the USA, on the whole, and even now, leans a bit more toward puritanical and/or Evangeical beliefs, or at the very least they're part of mainstream thought. If true, it could explain how so many Americans are wary of being alone with the opposite sex.
For example, I find that in my Western European surroundings casual hookups seem much more part of the day to day fabric than they seem to be to my American friends. It strikes me as not unreasonable to conclude that if casual sex is more acceptable here than in the USA (publicly, anyways), what constitutes crossing the boundaries of propriety in the USA might not be considered in the same way over here.
Specifically, I've worked in more than one relatively boring, conservative corporations where at the bigger parties various people 'hooked up' with various other people, some of them involving clear power-discrepancies. Nobody batted an eye and the whole thing seemed to be a 'what happens in vegas' type of thing, warranting some juicy gossip at most.
I'm not saying that this is how things should be, to be clear, just that generally my impression is that the USA is generally more uptight (best word I can think of, but I don't mean it disparagingly) about these things.