i agree with the book you've read that the word "kawin" is somehow impolite for some people in Indonesia.
I said some people, because Indonesia has many different culture, and the fact is that in some places in Indonesia,
especially those that was claimed as 'old-fashioned' and still so traditional, the word is still be used
but for modern people, we prefer the word "menikah" than "kawin" as it is more formal and polite
i don't know, but somehow in this time, the word "kawin" has a negative implicit meaning
that's my answer, came from my mind
sorry for any kinds of mistake or maybe my bad use of words
hope it'd help you =)