Im speaking here from a secular point of view, not from a religious point of view. Marriage is important in society because it is a status that gets not only legal recognition but support by the state. Is it not dangerous to you that a fruitless and sterile relationship is being supported and defended by the state? That it has equal status to a relationship(heterosexual) which is the basis for all of society? That people who have mental problems are adopting children who would otherwise be adopted by heterosexual couples? there are tons of negative affects it has on all of society and it only further exaggerates the moral decline in society. Where does it end? Ive read about multiple cases of parents starting relationships with their children. Should we allow them to marry as well. There are many atheists against gay marriage because it should be common sense to anyone that this kind of behavior is absurd and accepting it as good is just dangerous.
Also a few of the things you stated are just not accurate. Exclusive homosexuality is almost never seen in animals. Animals exhibit all sorts of sexual behavior. A dog will hump male dogs, female ones, humans, objects and then studies supporting homosexuality will take that and say there are homosexual animals. Only humans really exhibit “exclusive” homosexuality. Even Romans and greeks didnt have a concept of exclusive homosexuality. 5-10% of society is also too high. From articles ive read its more around 2-5%.