Homosexuality is not natural, much like eyeglasses, polyester, and birth control.
Heterosexual marriages are valid because they produce children. Infertile couples and old people can't legally get married because the world needs more children.
Obviously, gay parents will raise gay children, since straight parents only raise straight children.
Straight marriage will be less meaningful if Gay marriage is allowed, since Britney Spears' 55-hour just-for-fun marriage was meaningful.
Heterosexual marriage has been around a long time and hasn't changed at all; women are property, blacks can't marry whites, and divorce is illegal.