It may not be possible to legislate on controversial issues even when they are not legally binding - but I think there is another strategy.  Looking at the human race now and comparing it with what you might want an Intergalactic species of quadrillions of citizens to look like, I can imagine a questionnaire to screen people for suitability if leaving Earth in future.  Believing that euthanasia should be legalised would be at the top of my list!