I keep seeing Homosexual unions being put into the context of a Rights issue. I just do not see it that way. A friend of mine stated that Everyone DOES have the right to marry someone of the opposite sex.
It boils down to the definition of marriage. Which is about sex. It is obvious to all that sex is about reproduction. That’s what it’s for in animals, and that’s what it’s for in us. We may find it enjoyable, but from a biological perspective, that is motivation to get us to engage in it and thus reproduce our species.
Sex is about babies, and there is an important fact about babies: They are helpless and require an enormous amount of care and attention. It’s a full-time job more than one person can handle. Even when they grow out of the infant stage, children still need two parents to take care of them and provide for the family.
Children also take a long time to mature. They won’t be biologically mature for around two decades (if then), and they may not be socially mature and able to serve as functioning, independent members of society for even longer. When more children come along, that only prolongs the period of investment parents have to make in raising their offspring.
Raising children is a multi-decade effort that needs the involvement of both parents. The fact that human offspring require so much care and take so long to mature means that their parents need to be joined in a stable union. This union even extends beyond the childrearing years, because by the time the offspring are grown the parents are in their declining years and need to start taking care of each other (as well as receiving help from their offspring).
From my Catholic standpoint, as the Code of Canon Law points out, "marriage is a permanent partnership between a man and a woman ordered to the procreation of offspring by means of some sexual cooperation" (CIC 1096 §1). This is the reality of what marriage is and what it has been understood to be in all human societies in history, even those that have been otherwise tolerant of homosexuality.
Human nature thus leads to sex, which leads to offspring, which leads to the reality of childrearing, which leads to marriage—an institution found in every human culture and understood in the way just described..