I don't think deciding in absolutes is the way to go. Each family is uniquely different to the one before and you should not apply the same ruling for every case.
In case of separation, ideally both ex-partners should decide on their own how to rearrange their lifes. If they can't come to an amicable agreement, a neutral 3rd party needs to solve the litigation with that specific case in mind, not followjng a genericalistic rulebook.