To be going on with, spathi, here’s a brief description of some of the problems that may arise when parallel legal systems attempt to deal with the issues of marriage and divorce. In the extreme in can lead to participants indulging in bigamy. It is particularly pertinent in the sort of matters I described that “Sharia Courts” deal with. Alas I cannot provide you with a link as it is from a restricted intranet so you’ll have to take my word that it’s genuine. Sufficient to say it is among the guidance given to judges:
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Marriage & Divorce
A civil marriage or partnership must take place at a registry office or venue approved by the local authority. Whilst a religious marriage may take place in a church, mosque, temple or other place of worship, the relationship will not be legally recognised unless the place of worship is either Anglican or registered by the Registrar General for marriage. Divorce is similarly only legally recognised if it complies with legislation.
There are different cultural approaches to divorce:
• There are many different views on the acceptability of divorce in different cultures.
• In some religions, a woman cannot get a religious divorce unless her husband agrees. Therefore, although a woman might be able to get a civil divorce under the law in England and Wales, if her husband refuses a religious divorce, in practical terms she cannot remarry in her community.
• An opposite difficulty may occur where a divorce is granted by the religious authorities, but has not been effected under civil law. The individuals might remarry under religious law, believing they are free to do so. This could lead to a situation where, eg, a man is married to one women under civil law and another under religious law. This can cause complexities regarding the rights of the respective women.
• In some communities difficulties arise on divorce when the couple discovers that their religious ceremony was not legally recognised, as their place of marriage was not registered. Women in particular, more frequently the financially weaker spouse, are likely to lose out if their rights on divorce are only those of a former cohabitee, not a former wife. A woman may not even have realised until this point that she was not married according to civil law.
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Not that long ago (when the “status quo” in the UK was not quite what it is today) it would have been unthinkable that (official) UK courts would have needed to consider such matters when making their rulings. Today they do because parallel systems (which their adherents believe are "official") exist. That is why I say that the things I foresee are not as fanciful as you seem to think.