But this goes deeper. It is the conservatives in this case who are trying to change something, but they are unaware that they are doing it. They are trying to define marriage by a tradition rather than by the reason for its existence. Marriage exists as an institution of commitment to provide stable relationships for the purpose of promoting welfare in society and good homes for children. But conservatives wish to define it as a legal union between a man and woman. They do so in order to exclude homosexuals, whom they see as deviants and sinners, from having relationships with equal recognition in society. Hence, they have defined marriage as an institution of exclusion rather than one of inclusion and thereby cheapen it. Rather than marriage being about how loving, committed, and capable of providing a good home for children a couple may be, the factor that conservatives wish to stress is the gender of each partner.
Liberals aren't changing anything about the nature of marriage by allowing same sex couples to marry. Liberals are embracing what marriage is and are trying to protect it from being redefined by a tradition of procreation.