Because the bible speaks against it. The bible tells that men being with men is an abomination to God.
Because Religion is about controlling the masses and destroying individuality. It's about corrupt powermongers and unjustified acts of ignorant hatred masked by "Faith". It's the bane of Humanity and is holding us all back, promoting division and stupid rituals. Marriage is a social institution and need not be remotely religious, so anyone who dislikes that can feel free to comment and be freely ignored in response.
Which "religion"? Or has xtianity finally replaced every one of them? Did I miss something? Or did everyone just go to heaven and leave me and EP behind?
I can name a few that have better things to do than worry about which body parts a bride and groom insert into each other.