Aquí is mainly studied what marriage is. In passing, it shows how this debate has profound consequences for the common good and how its protection should be regulated. They systematically review the most important opinions on marriage, in favour and against, and then weigh up their consistency. The analysis is divided into two parts. The first defines the concept of genuine marriage and shows the consequences it has for the common good, which justify the State’s action in this area. There are all kinds of Roman, monognamic or non-roman unions and the public interest that they can present. The second part deals with the most common objections raised against traditional marriage, not before. These are objections for conservatism (why not extend traditional rules to the gay community?), pragmatic (what happens to the specific needs of couples?), equity (perhaps the marital conception of marriage sacrifices the full development of some people in favour of others), nature (is it not natural?), and neutrality (is it not true that traditional marriage laws impose controlversial religious and moral views on others?). The main conclusion is that marriage, understood as the marital union of husband and wife, actually serves the good of the children, the spouses, and the common good of society as a whole; these benefits do not exist in the same way in equal marriage.