They can’t. Legalized homosexual marriage opens the door to everything up to group marriage. If 10 men and 10 women all profess their love and want to engage in consensual sex, with all the benefits of a married entity, there is absolutely no basis to ban the practice once you say marriage is no longer one man to one woman.
Prohibitions on any number or type of marriage become indefensible as soon as homosexual marriage becomes legal. A Grandmother could marry her grandson.
A man could marry his horse.
That’s why the real target is destroying marriage.
Civil unions, along with common contract law and powers of attorney, satisfy the legal requirements of homosexual relationships giving them full protection less marriage.
It is the natural outgrowth of permanently immature sexuality seeking recognition (unattainable) and in the service of the evil one.