See my related essay: What is Special about Genetic Paternity?
Generally, before the government can take a person's life, liberty, or property, the person must be afforded due process - a neutral, fair process by which the appropriateness of taking the person's life, freedom, or property can be verified.
Indeed, it is a general moral principle that it is generally wrong to take away someone's life, or property, or freedom - you have to have a really good reason.
Sometimes, the reason is that the person agreed to forfeit property. This is why we enforce contracts.
Sometimes, it is "fair" to take away someone's life, or freedom, or property, if the person has done something that society sees as morally reprehensible. This is the principle underlying the criminal justice system.
Governments can even justifiably take property when a person has not done anything wrong, and has not agreed to give it up, as is the case with income taxation. The justification is slipperier here, but at least there are genuine moral justifications for taxation, even if their correctness is debatable.
But there is no moral justification for forcing a person to pay child support for a child he did not wish to conceive.
In most of the world, for most of its history, men have had reproductive rights, and women have not. In modern Western society, women have reproductive rights, and men do not. But "karma is a bitch" is not a moral argument.
It is wrong to force a woman to have a child when she does not want to have it. Simply consenting to sex does not, in our culture, entail consent to reproduction, nor should it. A sexless world is a miserable world; restrictions on reproductive rights are restrictions on one of the most pleasurable activities humans have access to.
Similarly, it is wrong to force a woman to have an abortion if she does not want one. The bodily invasiveness of pregnancy, and of abortion, should indeed mean that birth and abortion are under the exclusive control of the womb-owner.
But it does not follow that a man should have to pay child support for children he does not wish to conceive. In our society, if a man gets unlucky - a condom breaks, his girlfriend was lying to him about using birth control, his girlfriend was lying to him about not trying to get pregnant, etc. - and unwittingly functions as a sperm donor, he is completely at the mercy of his female sex partner.
Having an unwanted child is a personal and financial disaster. Child support laws should not encourage women to inflict this tragedy upon others.
Memento mori, as Chip says.
"Don't breed or buy while foster kids die"
On an unrelated note, I had a question about what I meant by my "don't breed or buy while foster kids die" spiel. "Breed" is obvious; by "buy," I mean utilizing expensive reproductive technologies such as in vitro fertilization (IVF) and surrogacy. It is as immoral to participate in such processes as a sperm or egg donor, or as a surrogate, as it is to participate as a prospective parent.