In other words, if someone is for gay marriage, then on what grounds can they forbid polygamous or incestuous relationships or incestuous relationships?
Of course, this assumes that these other less popular alternative lifestyles should be forbidden as well.
Examples: -Inter-racial marriage was once illegal in the US. Now it's not.
-Marriage was used as a tool to forge alliances between royal families in Europe. Now it's not.
-In Europe, it was once common for a bride's family to pay a dowry as a part of the marriage agreement. Now it's not.
My point is that elements of our collective cultures evolve all the time. The concept of marriage exists to serve culture that it's a part of, not the other way around.
It's discrimination because non-gays can marry and gays cannot.
This does not apply to polygamous or incestuous relationships. Those are not recognized by law, but they are denied to everyone equally.
Also, "union between a man and a woman" does cover incest so clearly that's not the definition used by courts when deciding who can marry. Your premise is flawed and even if it wasn't, you're just trying to confound completely separate issues.
Again, it's about discrimination.
However, the law discriminates against polygamous relationships based on the number of people involved and incestuous relationships based solely on who their relatives are.
The argument is, why is discrimination against gender bad, but discrimination against relatives is ok. If two brothers want to get married, why should that relationship be denied by the law?
Same logic applies to incestuous marriage, although this would be more extreme since there are very strong taboos against it (not throughout all history, though, e.g. in Ancient Egypt pharaohs mostly married their sisters).
You're arguing a straw-man.
+1 You spelled bestiality correctly.
Seriously, I'm not joking.
It is a valid point when arguing against gay marriage to point to the "slippery slope" of bestiality. But it is also a valid point to state that there's really no good reason to make marrying a chicken illegal. Fundamentally, I believe most opposition to gay marriage is based upon disgust. And I'm confident the same is true for marrying a chicken. Disgust is hardly an adequate justification.