Is marriage equality irrational

There is something I see as irrational in the debate over so-called marriage equality. Now if I were gay, Christian, and believed that gay marriage were consistent with my faith, then seeking to have a relationship recognised as a marriage makes sense.

But what if I reject religion, and specifically the Christian version of such? What, if in that rejection I see Chrisianity as inhibiting and repressive, putting silly, stupid restrictions on human freedom?
What then is my rationale basis for a monagomous relationship? 

When I look at both the world of humans and the larger world of living things, monagomy seems a rare, unusual sort of thing. There are few animal species that mate for life. And in the human sphere, monagomy seems to be something that largely evolved in the Christian world. Polygamy was allowed, and in one place even commanded, in the Old Testament and seems to be the norm in other cultures.

So, if the only basis for monagomous marriage is Christianity, then why would those who reject Christianity clamour for the right to relationship that appears only to have any basis in that which they reject?