The government does not- for the most part- define social relationship with respect to marriage. A few exceptions are prohibiting the marriage of people who are too closely related, those who cannot legally consent, and until recently, those of the same gender. The legalization of same sex marriage moved the government one step further away from defining social relationships so I really don’t know what your blathering about.
In reality, it is the people who are involved in the marriage that determine their social relationship though an implied, unwritten and often unspoken contract with respect their expectations of marriage and of each other within the marriage. Yes it is that simple
The government’s role is limited to establishing the legal protections and financial benefits for the couple and their children. Beyond that, government has no role in the day to day functioning of a marriage . You are inventing issues and problems and making things way more complicated then they need be.
Marriage does not need to be changed or fixed. We got it right by extending marriage as we know it to same sex couples. We made it more inclusive and as such strengthened it as an institution. The only thing that need to be fixed is the brains of people who cannot accept and deal with the concept of marriage equality. Equality made the lives of hundreds of thousands of couples better. It made the lives of their children better. It cost no one anything. Those who are threatened by changing social and sexual norms need to consider whatever issues they are having with their own sexual or gender identity, or in their marriage and get professional help if necessary.