Marriage is about a lot more than protecting the woman. It is a legally binding contract that gives each rights on behalf of the other. It was originally recognized by the state to help produce a family unit that appeared to be most helpful to the nation. Since, however, it has been used to control things like visitation rights in a hospital or who gets to control things if someone dies. A widow, for example, has rights to the estate unless the will says otherwise. A boyfriend/girlfriend has no rights to the estate and by default it goes to the closest family member(likely parents or siblings).
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DBeta commented on Ted Haggard's New Church: Gays Welcome, Not Gay Marriages