Historically I think a more important reason for the exclusivity of marriage was the man’s desire to know which children were really his—if his wife was not faithful to him he might be raising some other man’s children and passing on his inheritance to a son who wasn’t really related to him. The danger of venereal disease was probably much less a factor before 1493, when Columbus and his men returned from discovering the New World and brought back syphilis.
You have touched on another aspect of what I call the Natural Law of man’s biological union of marriage. The union between a man and woman is to insure that the children are raised to maturity. Infidelity is a threat to that union. It is another aspect of the Natural Law of that union.
STDs are the medical consequences of infidelity. Even though syphilis had not appeared until the 14th century, gonorrhea was spoken of by Moses. I am sure that Christ knew about gonorrhea. That protection from the exclusive union between a man and a woman had that protection from the Darwinian consequences of a STD causing infertility.
The Bible speaks about this Natural Law too. It calls the homosexual union a lie. That is the truth of the matter. The homo marriage thing, when in context of Natural Law, is a defect that when looked at in Darwinian terms, violates the principle of the selection of the fittest. Homosexual marriage selects the wrong sex, which causes no genetic transfer for a new human being. With no genetic transfer, you have end a genetic lineage that has had millions of years of perfection. It is tragic that it happens. Maybe we should take a sample of sperm (and ovum) from all homosexuals, to continue their genetics—for the benefit of mankind—for their misguided life’s choice.
In my mind, conforming to Natural Law has its perfect benefits. There is nothing that describes the total happiness that is involved in the love between a man and a woman—and their children. This is no lie: Nothing comes to that level of fulfillment that can be experienced between a man and a woman. Nothing.