Even an atheist knows you need a man and a woman to create a family. Where do children come from? Thin air?
Everyone likes to be rational, or desire to be so. Part of being rational is looking at what a thing is, and why it is as it is. The human body contains genitals. It is obvious that at least one of their purposes is to produce human life. Anyone who denies this is irrational.
So the question is: ought we to follow the rule of nature? According to dieticians, ecologists, physicists, engineers, and farmers and divers others, the answer is “DUH! YES!”. So why this exception with this very crucial part of human existence - reproduction and the survival of the species? Why don’t we have a license to genetically modify food or cause snails to go extinct, but we do have a license to mutilate a vital organ?
Anyone who supports gay marriage lacks critical thinking skills, or hasn’t applied them to this field - to his discredit.
Besides, if it’s imposing our religious beliefs, it’s sure the hell imposing more than Christian ideas.
Jews support this. Moslems support this. (We put “under God” on our money and in our pledge; that’s already something we and Jews and Moslems have in common.) It is not condoned by Buddhism, or indeed by most religions. Even the pagans never conceived such an idea.
It ought to be telling when not merely a majority of people, but a majority of religions or a majority of philosophies do not support an idea.