Not a good example because you can add hetrosexual and it works too. So you are allowing ALL of them to work. Don't add to the acceptance.
Which is why I don't use the "equal rights for consenting adults" argument. But if you do, you are morally and logically bound to support polygamy, incest, and gay marriage.
In my opinion, marriage is based upon three things: (1) protect the children from parental abandonment, (2) protects the wife from the her husband abandoning her, and (3) protect the husband by helping to assure his paternity of the children to which he is now committed.