Why does French hope that Rose is not his cousin?

At one point, French talks about the extended family as one of the aspects of life before the collapse of society. During that time, members of the tribes would often discover cousins that they had not previously met. Young people learned to ask about these family connections before becoming involved with someone new. Otherwise, they could sometimes begin dating before discovering they were
cousins. This explains French's attitude when he meets Rose for the first time and hope that they are not cousins.