The Family: A Proclamation to the World:

"God has commanded that the sacred powers of procreation are to be employed only between man and woman, lawfully wedded as husband and wife."

Concerning those who do not accept this pattern this scripture says:

"Wherefore God also gave them up to uncleaness through the lusts of their own hearts, to dishonour their own bodies between themselves: Who changed the truth of God into a lie, and worshipped and served the creature more than the Creator, who is blessed for ever. Amen. For this cause God gave them up unto vile affections: for even their women did change the natural use into that which is against nature: And likewise the men, leaving the natural use of the woman, burned in their lust one toward another: men with men working that which is unseemly, and receiving in themselves that recompence of their error which was meet."
Romans 1:24-27