I think the Roman catholics are right about not separating sex *for companionship and pleasure* from *sex for procreation*

The latter is the organic consequence - the fruit - of the former 

What God has joined together let no man separate asunder

If sex leads to fruit then thise who caused the child to be have a duty of care for that child. Can't escape nature. I forgot which libertarian had an excellent case for this based on common law and tenancy etc.