Cultures, ways of life, customs, religions and languages differ wildly from country to country. It is naive in the extreme to think that one rule of law and one Government (i.e. one country) can represent all of these differences in a peaceful manner. What you will end up with is the creation of extremists on all sides, and significant unrest.
The further apart groups of people are in the above mentioned categories, the less suitable they are to live in the same country, governed by the same laws. This is why, when demographics skew heavily in favor of one group of people in a particular part of a country (typically the minority group) we start to see demands of autonomy and secession.
It is fantastic to be able to experience broad ranges of cultures, customs, religions and ways of life. It's called going on holiday. While you are on holiday you respect the host's laws, customs and way of life, even if you disagree with them. Then, you board the plane and leave. There's really no need to try and cram every different way of life into the one country.
