Mathhew, the "ā" on "valā" makes it "and the man." Duolingo wants you to understand how the "ā" works, thus you were marked incorrect.
Isn't the answer "the man and woman are sleeping " technically correct? And wouldn't the addition of toghether improve the fluidity of the sentnece?
In this sentence woman come first. You switched the order, and Duolingo sanctioned you for being macho :D