wheresmypaddle - I couldn't see a direct answer to your question (apologies if there is one. It's a long thread!).
You are poorly protected if unmarried in a number of ways.
If your partner's assets are over the inheritance tax threshold, you will pay inheritance tax on them, whereas you wouldn't as a spouse. Other than the very rich, this is mainly relevant to people who fall into some or all of the categories of: older couples who have a lot of equity in their house; people in expensive cities who've paid of a fair amount of mortgage; people where a house is in one partner's name.
You have said you have a will. If you didn't, you may not inherit anything from your partner. LOTS of unmarried couples with children don't have wills. If the assets pass to young children or parents, it can make day to day life very difficult. The same would apply if a will was invalid for any reason (e.g. a glitch in signatures).
Many pension schemes will not give a surviving partner benefits in the same way they would a surviving spouse or civil partner.
You may find issues if there were a long illness or other medical decisions which your partner could not make for himself. There have been instances where long term partners are shut out of these decisions and the parents get to make them.
You do not qualify to apply for maintenance if you split (other than on behalf of dependant children). This may be fine whilst you are both young and working, but women often build up less long-term financial stability than men because they are more likely to work part time and take career breaks for family. A co-habiting woman who splits in her 50s with grown up children and who didn't work until they left home can find herself exteremly financially disadvantaged compared to a married woman in the same circumstances.
Does that help answer your question. I'm not commenting on the morality of it, or the public policy, but the fact is that, if you have any degree of financial stability as a result of one partner earning more than the other (usually the man earns more, but obviously not always), you are disadvantaged if not married. Likewise if you have substantial assets (house above IHT levels, one has a decent pension) and in situations of ill health. There really are lots of ways that it is beneficial to be married, but most of them are very long term.