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.
Absolutely, it is about discrimination. Gays marriage choices are discriminated against based solely on the gender of their desired spouse. As ridiculous as it is, the law allows a gay woman to marry any man that they want.
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?
Both gender and relationship to the potential spouse are attributes of a person that cannot be changed. Why is it ok to discriminate based on one but not the other?
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.