I told you chapter 15 wouldn't stop.

To provide some cultural context, in Amathae the age of majority and marriageable age are relatively young, and Amber has attained it (as alluded to on page 324), so this isn't a fantasy proposal. People in Amathae do sometimes get married this young (precipitated often by women's lack of rights, which we'll get into more soon).

However, out-of-the-blue proposals like this are indeed irregular and therefore it is a big surprise for Cygnus to say something like this right now.