Marriage is a bond between a man and woman. Are you saying brother and sisters are not law abiding citizens and should afford the same rights as gay couples?
In the case of incest, the progeny suffers from lack of genetic diversity. Although one outcome of marriage is the probability of off spring, in cases of incest and the resulting suffering, incestuous marriage should not be permitted.
Some contracts can result in bad outcomes. That's why each and every contract is not protected by law. If you contract me to spray your orchard with DDT, we would enter into a contract which would have bad results for more than both you and I. If, on the other hand, you contract me to spray your orchard with a safe and effective pesticide and I follow all the manufacturer's instructions and all state and federal laws concerning pesticides, our contract would then have the protection of law.
If a contract is beneficial to those parties in the contract and the greater society, there is no problem. What parts of same sex marriage adversely effect the greater society? What parts of same sex marriage adversely effect you personally? What harm is wrought to you?