It still surprises me that so many people just go ahead and create an entire new human without really thinking through what they are doing to that person. It surprises me even more that so many people seem to think that life is inherently good and that living is a privilege and a treat. I find that outlook very hard to understand.
My convictions come from a deep concern for human rights. This is how I look at it:
1) In most modern, civilized societies, negative rights are taken very seriously. Those who violate basic negative rights -- through murder, rape, etc -- are punished severely, both legally and socially. The vast majority of people agree that it's unethical to violate the negative rights of others.
2) When a person has a child, they are exposing the child to millions of different harms, and they know that the child will experience many of those harms over a lifetime. If you know that an action (procreation) will result in harm to another person, and you go through with the action anyway without that person's consent, you're committing a negative rights violation.
3) Some people will respond that the positives of life outweigh the negatives. However, this is irrelevant. If you hit a random man on the street with a baseball bat without the man's consent and then give him a thousand dollars, you've still violated the man's negative rights. Perhaps the man would have consented if you told him about the thousand dollars beforehand, but that doesn't matter; harming someone without their consent is a negative rights violation.
4) Conclusion: procreation is a negative rights violation, and it should be prohibited in civilized societies just like other negative rights violations.