So you are OK with brothers marrying sisters? Mothers marrying sons? Fathers or grandfathers marrying daughters etc? You are OK with multiple wives and or husbands (a situation that would certainly not be good for children)?
Unlike homosexuality, where procreation isn't possible between the couple, close relatives creates another situation all together. Inbreeding reduces the genetic pool, increases recessive gene expression, and generally is a bad idea, whether it be those of the lower animal kingdom or humans. You can expect many problems in reproduction between siblings, such as;
* reduced fertility both in litter size and sperm viability
* increased genetic disorders
* fluctuating facial asymmetry
* lower birth rate
* higher infant mortality
* slower growth rate
* smaller adult size
* loss of immune system function.
How does this relate to homosexual marriage by any form? One cannot reproduce thus poses no health risk to a child bore by them and one likely will exhibit many genetic anomaly that not only will effect the child, but the future generations coming from the birthchild of such a relationship's children. To even associate such a generationally dangerous with homosexual relationships is a pure diversion from the fact. It's apples and oranges, you cannot use a comparison of apples and oranges to express the difference between the flavors of various apple breeds. thank you, come again.
As for being good for the children or not, I don't believe bigamy is necessarily a problem for the children, on paper. The problem with bigamy and children is a deeper rooted situation that involves the obvious traits associated with the practice. Patriarchal idealism is core to polygamy, 1 man; many wives. The inverse services similar equality bias where one woman has many husbands. If man or woman were equal in such practices, then polygamy simply would NOT exist since to exist it requires a lean to either the male or female aspects of the relationship, giving one more and the other less. The problem this causes with children would be that in which the child would be indoctrinated in either the lesser or higher value of ones gender, where the poly gender would be of less value. While on paper it may be sound, so is communism, but the human nature of these are both apparent. Whether this should be legally sanctioned or not, is not an object for discussion here, I personally find polygamous relationships to be unsound not only for children but all involved if due only to human nature of jealousy and inequality of the gender involved.
You're argument concerning both of these marriage types, again is apples and oranges, Homosexual marriages show NO problem with children, how can they, they're unable to reproduce, your data concerning the parental abilities of homosexual couples thus should be saved for an argument whether or not they should be allowed to adopt, NOT marry, as that is not a concerning factor of the union.