The Pharisees wanted to know if Jesus believed divorce was legitimate for any and all reasons. The challenge is to translate the phrase, recognizing that it is a singular (each individual reason) and also general (any and all reasons).
Translators have to be careful with the nuances of the words they use. Did Jesus “notice” the other eleven disciples, or was be “looking” at them when he rebuked Peter? It creates an odd picture to not look at the person you are speaking to, and ὁράω can mean to “notice.”
There are all sorts of things worth pointing out in this verse, but the main one is how Greek can drop a direct object when it can be inferred from the context.
Every once in a while we come across a passage that says "you" and we can't tell if is singular or plural. When this really causes interpretive issues is when it is plural but English reads like it is singular.