There’s an episode of the Good Place where they discuss this exact thing (well, replace “immoral” with “romantic”, but still), and I’m pretty sure the motivations are the same. They don’t actually believe in determinism as much as they claim, but they don’t want to be responsible for their actions and determinism is a good excuse they can use. You can’t use logic to get them out of this belief, because it wasn’t logic that made them believe it to begin with.