Its really simple. Really.
Marriage has 2 definitions:
1. that sacred sacrament (traditionally) between a man and a woman before God. a priest reads from a bible, and you repeat dogmatic promises that you promise to keep, but don't (but that is a post for another day). Basically, its a church ceremony.
2. by law, marriage is the signing of papers that befit you and your spouse certain rights functional as long as you're married. They total about 1,400 rights.
Considering that you can become an ordained minister online nowadays, or even find some sympathetic priest to perform your union in a place of worship, definition 1 of marriage isn't a problem to obtain - you can get those anywhere.
It therefore follows that definition 2 is what is meant by Gay Marriage: rights pure and simple.
Sure they've been given civil unions, but these only deny them, oh , just OVER A THOUSAND RIGHTS given to hetero couples.
So what reasons to deny these adults these rights?