Why does Joe think mother will disapprove of Chris and Ann?

Keller notes that from Kate's perspective, Larry is not dead, so Annie is not available to Chris. But Chris insists that there is no other girl for him, even though they have not seen each other since the war. He declares an ultimatum: if his parents will not accept his marriage to Annie, then he and Annie will just get married and move elsewhere. Keller is shocked that Chris would leave behind the family business.