Duty to Breed
A letter by L. E. Bell in today's Rocky Mountain News expresses a similar sentiment on the procreation point: "By simply observing the marvelous and complex design of a man and a woman, it is obvious that the Creator (God) intentionally designed a man and a woman (exclusively) to be married, and to produce offspring."
The argument implies a religious duty to breed. It is behind the Catholic ban on birth control, the Mormon directive to bear as many children as possible, and the Protestant "Quiverfull" movement.
But, while raising children is an important part of many marriages, it is not a necessary part of marriage. Many heterosexual couples choose not to have children or cannot have them. They enjoy marriage for the romantic love, the partnership, the mutual respect, and the physical intimacy. Marriages that do not bear children are not, contrary to the suggestion of Renfroe and Bell, somehow of second-class or diminished status. Likewise, homosexuals can partner to enjoy the same fruits.
Moreover, while homosexual couples cannot get pregnant on their own, women can get pregnant using outside sperm, while men can adopt children (though I'm not sure how this works under modern American and Colorado law). So homosexual couples can raise children. (Is a homosexual couple that raises children superior to a heterosexual couple that does not, according to Renfroe and Bell?)
There is no good reason to claim that married couples have a duty to breed or that their marriage is justified by breeding. There is no good reason to claim that homosexuality is morally wrong (never mind a sin comparable to murder). There is no reason -- there is only religious faith based on an ancient book of mythology.