I haven't heard much discussion about future EU/UK relationships and how they are supposed to work in future, especially if you don't get married. Bizarrely, the widely acclaimed Renault advert is the closest I've seen to any acknowledgement of this issue.
All immigration of EU citizens (excluding those of Ireland) will be subject to same immigration controls as citizens of any other country. People can obtain "spouse" visas without being married. One of my friends (British) has looked into it for moving here with her partner and cohabitation is sufficient. I imagine its more difficult to prove a relationship is genuine if a couple aren't married or living together. There are financial requirements too.
EU citizens can bring a non EU partner to live with them in other member states with minimal requirements. EU member states are allowed to impose strict requirements on partners of their own citizens (as the UK has done), but not on partners of Citizens of other member states. So an EU citizen and British Citizen will be able to live together in 26 out of 27 EU countries + the EEA countries but not necessarily either of their home countries....!
) but i wonder if the greater number (i presume) UK-EU relationships than UK-elsewhere will mean the hoops you have to jump through (and expense) will become a much bigger and better known issue and there will be pressure on the Home Office to change things.