It is unacceptable to date your cousin.
Dave is hanging out with his cousin Lucy when she offers to be his secret girlfriend.
Dave wants to respond to Lucy's offer.
Dave tells Lucy that she's a pretty girl but that she's family and that sort of thing is forbidden so no.
Dave tells Lucy that he's always found her attractive and that he'd love to be with her.
Dave feels good for expressing a boundary for Lucy to follow and sticking to it.
Dave starts to feel regrets about his decision since they can't have children together, and he gets depressed.