This is a smoke screen from the real issue, which is equal treatment of our families. Convicted felons, prisoners and wife-beaters have been found to have the constitutional right to marry. So this right should not be denied to productive, loving couples just because they are of the same sex. Gay and lesbian couples would probably not be immune from the trouble and stress that befalls all couples. This is no reason to deny the many social and legal benefits of marriage.
Marriage is a heterosexual institution.
Making a safe and mutually-supportive family is not inherently heterosexual. Marriage is patriarchal. State marriage laws are largely blind to gender. What some perceive as sex-role stereotyping in marriage has nothing to do with marriage per se. It is a reflection of social attitudes, which attribute behaviours selectively according to gender.