In the UK - pretty much none. As there is no bill of rights or similar in the UK all law is based on precedent. With regards to property rights, if a 'common law' wife has not actually bought the property there can be huge wranglings in court as to whether, say, work she did for free in the family business, can be deemed to have monetary value and thus show contribution towards the house. The case law is very complex here and has lead to judgments both in favrour and not. Regarding any children, they will have rights. But if one of you died without out a will ('intestate') neither could get anything, whereas those legally married get everything. Regarding state benefits it may be best to be single, but in all other legal circumstances it really is against you to remain un-married.