I don't understand how giving two consenting adults the same benefits and authority as two other adults will irrevociably harm society.

If you want to keep the word marraige implicitly relgious I have no problem with changing the name to union.

I also don't understand the child arguement. Plenty of people have children whom should not. The fact that a man and woman can procreate does not mean they should. I don't think you should get benefits soley based on being able to perfrom a biological function