Why gay marriage should be legal in the United States.

So, this is basically the pro arguments from an online debate I was in, supporting gay marriage. I will remove the responses and add a few parts. That being said, lets go:
My Opening Arguments:
To begin, I will state my reasons for supporting marriage between homosexuals.

my first point is that there is no such thing as traditional marriage. Given the prevalence of modern and ancient examples of family arrangements based on polygamy, communal child-rearing, the use of concubines and mistresses and the commonality of prostitution, heterosexual monogamy can be considered "unnatural" in evolutionary terms.

My second argument would be from an economic stand point. Gay marriages can bring financial gain to state and local governments. Revenue comes from marriage licenses, higher income taxes (the so-called "marriage penalty"), and decreases in costs for state benefit programs. The Comptroller for New York City found that legalizing gay marriage would bring $142 million to the City's economy and $184 million to the State's economy.

Gay marriage will make it easier for same-sex couples to adopt children. In the US, 100,000 children are waiting to be adopted. A longitudinal study published in Pediatrics on June 7, 2010 found that children of lesbian mothers were rated higher than children of heterosexual parents in social and academic competence and had fewer social problems. A July 2010 study found that children of gay fathers were "as well-adjusted as those adopted by heterosexual parents ."

sources:
economic argument: http://www.comptroller.nyc.gov...
adoption argument: http://people.virginia.edu...

Civil Unions:
A civil union is NOT equal to marriage. One of the biggest differences between a civil union and a marriage would be that civil unions do not include federal benefits that are given to married couples. The Defense of Marriage Act (DOMA) issued in 1996 prohibits same-sex couples from receiving federal marriage rights and benefits.
Another equally important difference between civil unions and marriage would be that because civil unions are not recognized by all states, such agreements are not always valid when couples cross state lines. This isn't something that happens to people who have the right to marry.
Another example of inequality would be that now, after New Jersey's Civil Union Law took effect, at least 1 out of every 7 civil-union couples in New Jersey are not getting their civil unions recognized by their employers. That is 14 percent of civil unions. If 14 percent of married couples in New Jersey were being denied full, legally guaranteed marriage benefits by their employers, then people would be completely outraged, and there could even be riots in the streets.
And as far as adoption goes, under the current state of the law, many gay and lesbian parents are unable to assume full legal parenting rights, which results in children being denied health care coverage from their parents' employers. Children are also being denied survivor benefits when a parent in a same-sex relationship dies.

So, in conclusion, I believe I have responded to all of y opponents arguments, and have proved that Civil Unions are NOT equal to a marriage, in any sense of the word. I thank my opponent and the people who are voting.
Sources:
civil unions: http://www.factcheck.org...
DOMA: http://frwebgate.access.gpo.gov...

Maybe later I will post a response to common right wing arguments against gay marriage. Thank you for reading.