What if the issue was interracial marriage?
You should be asking yourself, "why do we have to keep revisiting marital issues?".
You'll eventually conclude is that marriage law is not about love or relationships. Instead, marital law is about regulating relationships that traditionally produced biological offspring, who inherited genetic traits from both people within the marriage. That's why it's illegal to marry your siblings. The presumption of paternity is why divorce courts are particularly hard on men, even if they aren't the biological parent.
Over the years, state governments and the federal government have violated Equal Protection to give special privileges to married people to offset their parental responsibilities. Violating Equal Protection has caused a litany of socio-economic problems for unmarried people. The public sector doesn't care. Economically-speaking, everyone cannot get treated with kid-gloves, and sympathy for parental difficulty is political gold. Discrimination persists because both sides support it, regardless of the rhetoric.
Liberals support SSM because it has virtually no effect on the socioeconomic paradigm in the US, and they are delighted to have an new micro-demographic to exploit for political power.
