God instructed Adam and Eve to multiply and fill the earth. I believe that has been accomplished.
As to legalizing gay marriages, that requires a total new definition for marriage, a definition that has NEVER been used in the recorded history of this planet. We live in an age where anything that can passed as law becomes legal. It is no a matter of right or wrong, but legal or illegal.
God instituted marriage to be a life-long commitment, heterosexuals have "legalized" no-fault divorce, a concept the Bible calls adultery, which is a sin. If you want to legislate Biblical morality, should we not start with divorce (take the beam out of the heterosexual's eye) and then concern ourselves with the splinter in the gay community's eye.