Marriage has never been a fixed concept. Allowing same-sex marriage is a natural evolution of the concept.
In the past, our definition and understanding of marriage included polygamy. Some societies that shudder at the idea of taking multiple spouses today once saw nothing wrong with it. The same applies to interracial marriage. At one time, interracial marriages were abhorrent and even illegal. Few hold that belief today. Our definition of marriage evolves and changes with public opinion. It makes sense that as attitudes to homosexuality change, so too should our definition of marriage.
Our definition of marriage can only be expanded when public attitudes force it to expand. This is not happening everywhere. There are many countries where public attitudes remain obstinately opposed to gay marriage and same-sex couples. Until societal currents force the definition to expand, the existing definition of marriage between one man and one woman must prevail.
[P1] The definition of marriage is not fixed. It changes with public attitude. [P2] Public attitudes have warmed to gay marriage. [P3] Therefore, the definition of marriage should evolve to reflect that.
Rejecting the premises
[Rejecting P2] Public attitudes have not necessarily warmed to gay marriage.