See below for my response about a parent marrying his child. Siblings getting married may be a result of childhood abuse. As such, it is best to not allow that to prevent abuse.
Or, how about two men and a woman or two women and a man?
Not my thing, but I am not about to force my will on others.
How about a father and his daughter? Or, a father and his son?
This borders on sex with minors. A parent who marries his children may have abused them as children (i.e. raised the child to be his sex-slave). This hurts the child.
How about a man and his goat? If a man is in love with his goat, who is to say that such should not be sanctioned? It won't affect you and it won't affect me. If they are happy together, why not? It is not animal abuse. After all, you can kill a goat and eat it. Can someone stop you from having sex with it?
It is animal abuse. While it is legal to kill a goat, it is not legal to torture it. Since a goat can't provide consent to have sex, then a man marrying his goat is technically raping his goat every time he has sexual intercourse. Since sex is implied in marriage, then marriage between humans and animals should be illegal.
I think that you missing some of the argument.
The point is that, if the definition of marriage is changed by one group, then the definition of marriage can further change. It is going to be at the whim of a judge or interest group.
If the law allows marriage between gays, it is discriminatory to not allow others to have their own definitions of marriage. What gives gays the right to change the definition to fit what they want but others are denied their definition? Gays are being discriminatory.
Let's change it to a woman and her German Shepherd. If the male animal initiates sex, she is not forcing herself on Wolfgang. She would make the argument that it is clearly not animal abuse. Wolfgang is happier, he eats better, his coat is shinier, etc. Do you think some sicko won't make the argument?
If there is no history of abuse, why can't a grown daughter marry daddy? They will make the argument.
DFU: Or, how about two men and a woman or two women and a man?
PSYCHO: Not my thing, but I am not about to force my will on others.
You are part of the problem. Retaining the definition of marriage that has worked for centuries is not forcing your will on others.