:roll: Here we go again...
Why not two men and one woman? (or visa versa)
Answer:
Monogamy reduces major social problems of polygamist cultures
This applies to polyandry also which is what your example is.
Why not brother and sister?
Answer: In otherwords incest....Children born from incestuous relationships are more prone to both mental and physical problems.
Father and son?
Answer: Not allowed because a father (or mother) can subtly coerce their child into things like this. Making them think that it is OK and perfectly natural for daddy or mommy to be boinking their child...and this kind of teaching can last well into adulthood and even until they die. And if it is instead a "Father and daughter" or a "mother and son" type of deal then you again go into incest along with coercing.
Man/woman and animal marriage
Answer: The most basic ability in order to go into a marriage is that it MUST be consentual. And animals cannot consent.
Man/woman and child marriage
Answer: Not only is that pedophilia but again, a child cannot legally consent to a marriage because they are not mentally capable of doing so.
These are just simple, basic common sense answers. If you want more detail then I would suggest doing your homework and looking this stuff up yourself.