They can be persuaded by teaching them history and biology. The fact that marriage has certain characteristics that are universal regardless of culture or religion is a reflection of the biological basis of marriage.
The fact that a lot of young people really think the purpose of life is to indulge every desire (especially sexual), while avoiding all responsibility, does not change the fact that there is one reason any living organism exists. That is to produce young to keep the species alive. Gay "marriage" cannot exist, because the biological function of marriage simply is not there.
>>> They can be persuaded by teaching them history and biology.
Think about the body’s sytems. God gave you a nervous system, a circulatory system, an endocrine system, a skeletal system, a digestive system, etc.
But He only gave you 1/2 a reproductive system.
Think about it.