남편 두 명이 어때요?
why is it translated as "A wife and husband are a couple" whereas the other sentence is translated as " fathers and mothers are parents?"
It's because fathers are parents and mothers are parents (individually) but 부부（夫婦） is literally husband+wife. "Couple" is a misleading translation though.
So is 부부 specifically for married couples? What would an unmarried couple be called?
Maybe "결혼하지 않은 부부"? It's a literal translation of "unmarried couple".
Provided that they are married to each other. I prefer "A wife and her husband are a married couple."
wife and husband are spouse is wrong? duolingo is dumb