My feeling is that the numbers are skewed because of the bias in the people choosing to report their relationship status. For the countries with a low number of reported single people, perhaps there is some stigma attached to being single?

numbers may be skewed, true. but in aggregate truth will out. perhaps some people are not truthful in their reported data (any data anywhere), nevertheless all you need to do is make assumptions on the veracity of the data and what you get is a +/- range from your starting point. that said, these numbers are kind of what you would expect if you just had to guess or rank.