Although one's personal relationships are a private affair, marriage is more than simply a relationship, as it involves a legal contract between two people. On the one hand, one can argue that the person with whom one chooses to have a relationship is a private choice, as are the terms of that relationship. For example, whether one marries someone of the same or of a different gender, and the race, ethnicity, or religion of one's spouse, is a matter of personal preference.
There are circumstances where marriage becomes a matter of public interest. The first is where children are involved. The second is the question whether both partners in a marriage are of the age of consent and are freely joining in marriage and are free to end the marriage. Next, the issue of division of property is a public and legal issue. When a couple, especially one with children, divorces, the question of how they will contribute to the care and support of their children is a matter of public interest.
Thus although much of marriage is a private affair, there are compelling reasons for public input into some of the conditions of marriage and divorce.