The government has an obligation to protect the life and liberty of the subset of human beings we call "Americans." If you commit a crime that obligation changes, of course, since the government also has an obligation to protect the rest of us from those who would do us harm.
Well, I consider a fetus a human being. It has done no harm, nor has it committed a crime punishable by death. More important, I don't like it when governments start getting clever about who counts as full human beings and who doesn't (See: Slavery, U.S., or Holocaust, Nazi). There are few areas where a bright line is more vital or necessary. (I'd bet it won't be very long before science is able to tell us whether some fetuses will grow up to be gay or not. The politics of abortion will suddenly get more interesting, I suspect.)
But once you're born, and -- hopefully -- properly raised, the government's chief obligation is to stay out of your way -- whether you're straight or gay -- so you can pursue happiness as you define it -- not how, say, Michael Bloomberg or Pat Robertson define it.
Which brings me back to gay marriage. Opponents of same-sex marriage insist gays have the same right to marry a person of the opposite sex as anyone else. It's a clever line, but it overlooks the fact that romantic love has been the paramount reason for marriage for quite some time. Telling people they're free to be unhappy isn't all that persuasive.
The whole point of the American way is life, liberty and the pursuit of happiness. So, come to think of it, maybe gay marriage and abortion have more in common than I thought.