Doesn't same part of the constitution that the defenders of gay marriage are using?

Technically, yes.

Marriage is a CONTRACT in the eyes of the law.

This implies consenting legal persons... bellow a certain age and outside of the human specie, it doesn't apply. According to the US constitution, the rights of every citizen or person naturalized within the US shall not be abridged (that's the 14th amendment): pets and trees are not persons, it doesn't apply. Secondly, age is also a factor. What is the age of legal consent in the US? It's not 5 years old, so far as I know, since pedophilia is illegal... kids don't partake to that group either.

At least half of your argument rest on a fallacy: it's a false analogy. In the US, gays are, insofar as I know, legal persons and they are, provided a certain age, legally enabled to enter into civil contracts like anyone else. It's not the case, I repeat, of pets and trees.

Now comes the question of marrying a relative -- that is, a member of family. Biologically speaking, it entails important risk, in terms of procreation -- which basically can't apply to gays, by definition. That's a sufficient rationale for me.

Recognition of any marriage, tax breaks and all the rest gives a Privileged Class to those who are married....in a nation of equals this is wrong. Give marriage back to the church. If the Catholic Church will not perform a same sex marriage and the Pagan Church of the Greenwood will then so be it. The government recognition leads to an elite class with privilege over singles and those who do not want to be bound to another and that is not "Equal under the Law".

The US Constitution limits the power of the government to only which is STATED in the US Constitution. Marriage is not stated so it is out and is an over reach of the government into the personal lives of "We the People"

As for pets and trees, I'm pretty sure neither is guaranteed any Constitutional rights whatever, unless there is a secret 29th Amendment granting rights of citizenship to Maples or something like that.

As for relatives, there is a public welfare reason to prohibit excessively close adult relatives from marrying one another.

Trees and pets don't have constitutional rights to marry.

Cousin marriage is already legal in several republican states.

A pet and tree cannot consent.

yes it does. I think that I will propose to the very shapely Blue Spruce in my front yard.