You may try to justify gay marriage all day long with appeals to forms of discrimination in days gone by, or because two people (why limit it to two?) "love" each other, or it's a civil rights matter, or whatever.  But, as I posted elsewhere on this site, you're not done yet.  You still have to explain why--assuming you're arguments hold--several people can't marry as a group, why a parent and an adult child can't marry, why I can't marry someone already married, etc....let's just change the laws on those choices, too.  Alright with you if any of that goes?

I don't have to justify it. Two consenting should have the right to marry if they choose to make that commitment.
As for people marring their children, well in times past that was an ok thing. I couldn't imagine doing it but then again everything is not about me now is it?
Gay people should have the right to marry their partner and get the same benefits as any other married couple. Gay people make good parents and good neighbors.
Which would you prefer: a married heterosexual couple in which the male beats the female regularly or a loving married gay couple living next door to you? Funny how one is accepted in our society but the other is not.
Who should have the right to define marriage? What is fair?
And I have no problem with polygamy. Right now I could use another husband , the one I have is laid off. What if he wanted another wife? Great, someone to help clean this house.
Listen, marriage is a personal thing and should only be the business of those people involved. The divorce rates among heterosexual couples are staggering, so it's a cinch marriage is not considered a "sacred institution".
In many countries the things you site such as polygamy, marrying your relatives are perfectly acceptable. Does it make us any better because we don't except those things? Does it make us a better or stronger country by denying gays their freedom to chose whom the marry?
Do some research on countries which allow gay marriage. Denmark for example allows a registered partnership and the people there are among the happiest in the world.
Progressive countries seem to be doing well, America needs to progress.