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This is the common slippery slope fallacy. Logically, either gay marriage is wrong in itself, and thus we dont need to discuss incest and polygamy, or gay marriage is right but leads to undesirable consequences, in which case we shouldnt be discussing gay marriage, but the consequences.
Now, as far as the analogy goes, it is nonsense any way you look at it. The fact that all three are social constructs do not make them equivalent. Incest, for example, is a taboo in most known societies, even those that tolerate or even fully accept homosexuals. As you pointed out, there are a number of different consequences to it, from the genetic, to the impact on family structure, psychological processes and so on. And what is more, several forms of incest are already accepted. It is only the more extreme, and thus damaging, forms of incest that are taboo (intergenerational, siblings, etc). Intragenerational incest that doesnt not include siblings has been accepted for a long time (i.e., cousin with cousin is a relatively normal occurrence).
With regards to polygamy, it would be a wholly different type of union, one that from every aspect would change the legal aspect of marriage in many ways. There would need to be significant changes in how property, child rearing, legal spousal rights, inheritance and even divorce rules are managed. Now, some might defend this, but regardless of that the fact remains that it would require some pretty sweeping changes.
Meanwhile, gay couples are a reality, are no more a taboo in several places, would not require any significant changes in the legal aspects of marriage, would not alter family structure, etc.
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