Simple secular case against abortion

In a recent article by Jonah Goldberg, I read how one can't lump all the social issues together. Some people are pro-choice, yet pro-same-sex marriage and some are pro-life and against same-sex marriage, etc. He then went on to write about how he has become more pro-life and why.

Excerpt:

As for abortion, my migration has less to do with religious arguments and more to do with my growing distrust of the government. Who is and who isn't a human being with unalienable rights is just about the biggest question there is. And just because the answer is usually obvious -- that guy, not that fly -- only makes it more important.

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).

This is a simple case against abortion, but just because it's simple doesn't mean it's not effective or that it doesn't cause one to think about abortion harder. Usually secular cases against abortion deal a lot with scientific studies, arguments and philosophy. Not this one. Although, after thinking about it some more this simple argument does assume from the beginning that the fetus is a human being. If that is proven then the rest follows.

My case is simple too I think and favors Goldberg's.

If the fetus is a human being then the fetus has natural rights of life, liberty, and property that are to be respected by fellow human beings due to natural law. If the fetus is an American then those rights are protected by the government. Abortion violates the natural right of life of the human being who is in the womb of his mother. Therefore, abortion is wrong according to natural law and illegal according to civic law.

The question: is the fetus a human being?

Recommended reading
Is the unborn less than human?
A secular case against legalized abortion 

10 arguments in favor of pro-choice policy