I feel like this is just an extension of the “my child, my property” mindset that republicans have. Sure, like others have argued, there might be cases of 25 year-olds genuinely falling in love with first cousins and the whole goverment-shouldn’t-regulate-love thing; but the vast majority of these cases are going to be home-schooled together groomed kids who parents fear having romantic relations outside the family might introduce them to non-conservative or non-religious viewpoints which might break their narcissistic control over their kids lives.