Which argument do you want to hear?
I could give the religious arguments, but everyone tells those, so I am sure you have heard them before.
Instead I would like to try to argue without religious connotations.
First of all, the definition of a marriage was a contractual agreement. An arragement between families for property, money, and children for family continuation and power continuance. Taken this historical reference, gays cannot provide the children for the family continuation.
The argument that homosexuals can have babies is also a new "fad". It is a 20th century invention with artificial insemination and organized adoption procedures. This does not or should not change the historical reference and definition of marriage because of scientific improvements and differing social mores.
But even if you look at today's definition, Webster's definition for what it's worth, marriage is defined as between a man and woman.
Looking into countries that have had gay marriage legalized we can also see interesting traits. Granted these countries haven't had gay marriage long enough to have tremendous data on the subject, but some trends are forming.
In the first country to legalize gay marriage, homophobic attacks are on the rise.
http://www.advocate.com/news_detail_ektid66850.asp
Gay marriage adds to visibility and therefore paints a target for understandably violent and intolerable people to attack. Businesses have been vandalized for those that cater to gay marriages and/or relationships.
Then there is also the tax credits and such the countries would be losing because jointly filed taxes and other tax breaks as well as the increased medical care needed for caring for spouses. All of this drains resources for other programs like education and feeding homeless.
With regard to Prop 8, this hearing was to discuss whether the proposed (and eventual passing) proposition was a revision or an amendment. It had nothing to do with saving marriages or about promoting gay marriage but rather a clarification as to what type of proposition change supports a revision or an amendment. In this regard, rights were being clarified from a previous amendment, just one other amendment. By CA law, since this isn't a large modification of their Constitution, this wouldn't classify as a revision that needed 2/3 vote from legislation passing before it got on the ballot. This is also not taking away any rights or laws or privileges away from a group of people. Also, this was exactly an exercise of CA voting procedures to modify using the power of vote to clarify and modify by the majority of the population the constitution, using constitutional means and in accordance to proper law.
I am busy at work but I wanted to write some ideas down and I can get into details later if requested.