Why is the assumption that for a relationship to be successful you must be married? Why is there an assumption that being in a relationship should lead to marriage?
For a lot off people marriage is an outdated institution where one person has to eventually succeed to the will of another, however obvious or not. The can lead to reductions in career success, personal frustration at the lack of control of one's own future and even being the support for the other person who may be seen as the centre of the relationship. For some it's an unnecessary legal commitment that they don't wish to impose upon themselves as independent adults yet for other's it's the legal security they need in today's society, certainly within the UK, should the relationship fall apart after children have come along.
Marriage is rarely an equal/equitable arrangement and is it really necessary in today's society where equality prevails when single anyway?