Liberal Controversy

It seems so strange for me when people think cliche thoughts and claim to be open and liberal. For example everyone who wants to be modern and independent thinking must fight for gay rights in our time …However not as many are eager to fight for the rights of those who practice polygamy. Though everyone agrees that biologically a gay couple doesn’t have any future on their own.¬† And on the contrary¬† the polygamy was / is a traditional way of building families with offspring for many cultures. Muslim, Jews and Mormons are all can be part of this ancient practice. Why then should we at first accept the law regarding gay marriages instead of trying to work it out with something that is acceptable for many cultures and seems to be more traditional in nature?