YANBU.
If people wish to cohabit, there are ways to set up their financial affairs to reflect their wishes to act as though they are married eg placing partner's name on deed to the house, arranging property ownership as tenants in common as to 50% each, ensuring that they have a valid will in place, contracts regarding any debts/assets etc.
I really do feel that providing cohabitees rights as though they are married will lead to a lot of confusion eg what length of time must they cohabit to attain this position in law, what happens if they break up at some point and reunite (will such rights continue, or will this create a new starting point in law?). What if one cohabitee is adamant that they do not wish to be treated as a married couple? Does that person have to "contract out"? If they break up permanently after 2 years, will such rights continue in some form or another, especially if there are children involved? If each cohabitee has children from an earlier/different relationship, how will these children be treated under the law if either cohabitee dies intestate? Will they need to have been adopted by each respective cohabitee to inherit from either estate?
Too many questions, I find, could potentially lead to a mini-deluge of litigation (considering the increase in the number of people who cohabit) that the courts can ill-afford. If people wish to have the same rights as married/civil partnership couples, there is an easy way to achieve this - enter into a marriage/civil partnership.
So many people argue that they do not want to be party to a "religious" institution, but that is a fallacy. Marriage/civil partnerships are no longer necessarily religious institutions (unless one decides to be married in church/receive a religious blessing) - at the basic heart of it, they serve as contracts to reflect one person's intentions as to another.
Marriages and civil partnerships also do not cost the earth - all you need is to register your intention to get married/enter into a civil partnership no less than 15 days before the date you wish to do so, and this costs £30 pounds per person. The ceremony at your local registry office will cost £40, and a marriage/civil partnership certificate costs £3.50. This means, in total, a marriage/civil partnership need cost no more than £103.50.
I say all this as a former cohabitee (now married). At the time, my partner and I arranged our affirs to reflect that we chose to live as a married couple (having valid wills which we renewed on a regular basis, setting up properties bought in the UK as "tenants-in-common" as to 50% each, ensuring any loans taken out were placed in both our names etc, placing our names down as next-of-kin to one another whenever either of us was in hospital, having a drawn up, witnessed and signed document stating that we considered ourselves to be next-of-kin and had the first/final say in the event of emergency/unexpected medical treatment and the person in hospital being incapacitated etc). I will admit that it wasn't easy trying to juggle all these things, so we did the next rational thing and got married to remove any doubt or possible confusion.
(Can you see that I've thought way too long and hard about this? )