Based on this week's theme--terms related to royalty--the post for morganatic does not seem to fit. There is no connection to royalty made in the etymology or the usage example. (Marriage between classes more often does not imply royalty + commoner, IMHO.) If in fact is related, can anyone explain how?

I guess it depends on how strictly you expect the theme to be applied. It isn't exactly a huge stretch.

The definition says "...the spouse of lower rank and the children do not share the titles...".
I'd say titles is the operative word linking to nobility. Offhand I can't think of any non-noble titles which might pass to children (or spouses) via marriage.

A morganatic marriage was one option considered as a solution to the Wallis Simpson affair.