Lewinsky was an entrepreneur post-scandal and got a graduate degree from the LSE. Clinton is a Rhodes Scholar, governor, and president.
Petraeus is a decorated general and a Princeton PhD. Broadwell is a West Point grad, Harvard doctoral candidate, and author.
I'm just saying - they all did pretty well. Normally we think of assortative mating in terms of relationships with commitment mechanisms so that long-term benefits can be extracted. What's interesting here is that that was not what these relationships were, and yet there's still a highly selective dimension to it.
It could just be a matter of opportunity, of course. Generally speaking, accomplished people hang around the White House and Langley. But it's also possible that this is not so much a "rational choice" as it is an evolved tendency (because if it were a rational choice you presumably wouldn't see it as much in these more tawdry affairs).