There's a range of issues / questions being conflated into one discussion
- This particular bereavement benefit, which is for the benefit of the children of the deceased, and making this conditional upon the marital status of their parents. The decision to marry or not is between the two parents, and any ramifications of their decision should impact directly on them, not their children, IMO
Then we come to the other rights and responsibilities of marriage - which may or may not be perceived as benefits to an individual, depending on their own situation
There seem to be two issues being conflated
- The branding issue
Those who wish to formalise their relationship in the eyes of the law, and sign a legal contract that would afford the same rights and responsibilities as civil marriage, but which is called something different - so not-marriage marriage. Whether this be civil partnership, legal togetherness or sparkly unicorn union, it's the desire for the ability to make a legal declaration of legal union between a couple, that isn't through the mechanism of what is termed 'marriage'.
- The opt-out not opt-in issue
Those who believe that couples should be afforded common law marriage rights by virtue of long term cohabitation and the raising of children together, without having to legally opt into a formal legal contractual arrangement. This case is generally made in order to offer the rights and responsibilities of marriage to couples who either weren't aware that their decision not to opt-in to a legal contract left them without the rights and responsibilities afforded to those who did choose to opt in. Or those (usually women) who would like to opt into a legal arrangement, in order to access these rights and responsibilities that would offer them legal and financial protection, but whose partners refuse to do so, leaving them exposed. This then brings into play all the associated issues of taking away couple's rights to cohabit freely without opting into said contract, and having a contractual arrangement imposed on them by default.
The first directly affects the child, not the parent
The second set directly affects the couple (irrespective of whether they have children). Any children born of the relationship may be indirectly affected by the decisions made by their parents, but the legal and financial impact of this contractual arrangement is on their parents.