Many defenders of family values, especially older ones, tend to think like this: marriage did not include gay couples a few decades back, and family values functioned well. We want to go back there. Therefore, we must oppose gay marriages.
However, this is a faulty argument. I do acknowledge that the decline of family values since the 1960s is a sad thing. However, there are many reasons for that, and gay people are not part of the reason. There is simply no way 2% of the population can cause or significantly contribute to a society-wide decline in family values. Things have gone wrong in mainstream straight society, and it is there things must be fixed. Scapegoating gay people and gay rights will only blind us to the real problems that need to be fixed.
Meanwhile, things have changed. In the past, many believed being gay to be a lifestyle choice. This is because they haven't seen many gay people around them. When everyone around you is straight, many people naturally believe that being straight is the only natural way of being, and being gay must be a choice some people in a very different segment of society make, for twisted reasons. In turn, this homophobia has kept gay people closeted and underground for centuries. Today's younger generation did not grow up this way, however. They see gay people everywhere in their lives. They have gay friends, and often have gay family members. That being gay is a characteristic of a person and not a lifestyle choice is a clear thing for the majority of the younger generation, and gay couples today live openly amongst us.
As the situation has changed, we cannot just hope to go back in time by refusing to accommodate the new situations. Excluding gay couples from marriage was just natural in the 1950s, as gay couples were generally closeted and excluded from every sphere of society. Excluding gay couples from marriage in the 2010s when they clearly live normal, integrated lives amongst us is rightly seen as a form of apartheid by many young people. Withholding rights and respect from gay people in the 1950s was just natural, as it was accepted that they have a deviant and criminal lifestyle choice. Withholding rights and respect from gay people in the 2010s feels like an injustice to many people, because it is hating people for who they are. In the 1950s marriage was a society-wide brand even as it excluded gay couples, because gay couples were out-of-sight, out-of-mind anyway. In the 2010s, if marriage excludes gay couples, it is by definition no longer a society-wide brand, and therefore would be seen to be very optional even amongst straight couples. In general, if we continue to hold the line that marriage should exclude gay couples, marriage itself is being demeaned in the eyes of many people, particularly younger people. This is clearly not a good way to encourage family values.
A re-implementation of the 1950s playbook is not feasible. The 1950s never will be again. But what we can hope for is a future implementation of family values that will make the whole concept popular society-wide again. Marriage equality is part of the work that will get us there. It renews the idea that marriage should be a society-wide brand, and that it is about family and commitment, rather than exclusion. Without embracing marriage equality, the public image of marriage can never be healthy enough for us to successfully then embark on further campaigns of marriage promotion and public conversations on marriage, which are what will bring in a new golden age for family values.