This is an antithesis because the phrase are opposites of each other. An idiom is using symbolic language to represent something. A pun is a play on words. An onomatopoeia is giving inanimate objects human-like qualities.
In my opinion, in the old days, everyone was jolly and grateful for who they got to marry. Money wasn't an influential cause, and although people were different colors, and had different personal attributes or aspects, they would get married for who they were deep inside. When Romeo and Juliet took place, they both were in separated families, and mainly people married their own color. However, money is greatly involved in our current era. People don't look for personal aspects or the way they are inside, many people are what we call "gold diggers". Only getting married for that cash, which is pretty sad these days. Another difference, in my opinion, is that people stayed married longer in the old days, not because of aging, but because of who they were inside, and the true gentleman inside of them. Today, many people get married just for the benefit of finance, and usually leave after cash is earned, which is overall sad.