I think it really depends on your definition of “marriage”; be it a socio-legal institution, which in effect amounts to a tripartitte contract between parties, traditionally a man, a woman and the state, or is it a moral/spiritual institution, a long term committed relationship entered into voluntarily by people who, because of their affection for one another, wihs to pool resources and share the joys and burdens of life.
You open with a debate about whether homosexual marriages should be allowed, but after reading your post, i think that you are grouping more towards a definition for marriage.
Before we “redefine the traditional ‘family’” as you put it, perhaps it would be better first to establish the idea of a traditional family and question why it came about in the first place.
If, by traditional family, you mean a man, his wife and his offspring, then you are excluding homosexual marriages from all of this. By definition, you’ve answered your own question. Gay marriages won’t be allowed because they are radical and anti-traditional. Then, there is no debate.
If, however, you are mooting the evaluation of gay marriage as a social construct and its feasability, then i must ask you for your definition of marriage.
Anyways, “will gay marriage lead to other types of unions” is a different question from “should gay marriage be allowed”, though the two may be linked. Your assumption is that other types of unions are inherently bad - morally repugnant. I ask you to revise that assumption, however.
What is wrong of marriages to multiple partners, if all parties are willing? Certainly the state does not have the right to prevent the getting together of individuals should they so wish. And as BrightNShiny pointed out, marriages to minors are allowed; in fact, marriages between minors is oft the norm in rural provinces of China.
Your last example of incest is a little tricker as there is hidden bias. I feel you are more value-judging the act of incest (which, as i agree with you, is reprehensible due to many reasons); which is different debating whether blood-marriages should be allowed.