I think that the core issue should be clear: either you believe that homosexuality between consenting adults is immoral behavior or you believe that homosexuality between consenting adults is moral behavior. An attraction initiates a connection after which a physical realionship emerges which grows to an emotional bond and ultimately love -- this is the approximate process for folks who are attracted to same gender partners as it is for folks in opposite gender partnerships. Either you think homosexuals are wrong and immoral or you think they are morally and rightly expressing a normal, human, biological imperative.
Logic flows that if you think homosexuality is wrong then you should be opposed to gay marriage. However, If you think homosexuality is morally sound then you should favor same sex marriage. In fact, the only position that I cannot tolerate is one that states that homosexuality is OK but just keep it away from me and my children and don't destroy the sanctity of marriage by allowing these folks to marry. Inherent in that type of statement is the underlying core belief that homosexuality is morally incorrect -- you are NOT OK with it. Let's at least be intellectually honest about the argument.
Marriage vs. civil union is not a semantic argument as far as I am concerned. I really could not look at q gay/lesbian American and say to him/her that on the one hand I have no moral dissonance with regard homosexuality but, at the same time, I'd like to deny you the right to express your love and commitment in the same why I can.
'Marriage', as a word/concept in our society has very powerful meaning. It is the one word that expresses the highest level of a person to person commitment that is available in our society. Imagine the friend that tells you they broke up with their girlfriend after 3 years of living together compared to the friend that tells you he is getting a divorce after a 3 year marriage. The difference matters. It is not just that there are legal ramifications, but there are social/emotional/societal ramifications that are inherently different (not to suggest a 3 year break up without marriage isn't sometimes traumatic -- of course it can be). But, nothing rises to the level of marriage in our society and the word itself contains a palpable message that no other word or concept is equal to. I simply cannot look my gay or lesbian fellow Americans in the eye, as I stand next to my wife and say 'sorry, you aren't entitled to society's highest acknowledgement of commitment to each other -- but I am'.
I believe that it should be the right of 2 consenting adult gay men or lesbian women to be married in America.