I'm thinking despite my tremendous writer's block that comes every now and again (read: when I'm depressed and my concentration suffers badly) I can probably write a little more, but in the form of lists.
A condensed version of antinatalism
1. Risks to child.
1.1 Immoral to gamble on someone else's life
2. Causes eventual suffering. By Do No Harm principle in human morality, this is wrong.
3. We do not notice how much pain there is in life. In actual fact there may be more pain than pleasure, due to boredom and tiredness due to constant school or work. If this is so, then from a purely utilitarian point of view, life creates more suffering than pleasure.
4. Pleasure does not matter at all in the equation because non-existent people are not deprived of any pleasure, so there can never be a reason to create a child.
4.1 Non-existent people turned into existent people do not benefit from pleasure (they were not deprived of it), but at the same time are hurt by pain, since before existing they knew know pain, figuratively speaking.
5. There is no duty to cause pleasure in others. Clowns are not heroes because they make people laugh. It is not required that you spend one hour each day causing someone pleasure by your morality, though to stop someone suffering is a different matter. We do not believe an action is moral intuitively that causes pleasure to others, that's merely a neutral thing - it is not immoral to neglect to cause another person pleasure if you can. An immoral action on the other hand is one that causes suffering to others - and neglecting to stop suffering when you have the chance (such as if you see a person about to be hit by a bus, and can save them with no risk to yourself) is generally immoral. Therefore it can never be argued that we should create people because making happy people is a duty.
Reasons pets are better than children
1. They stay cute forever
2. Won't grow to hate you unless you do something seriously wrong
3. Rebellious phase lasts at most a year
4. YOU AREN'T BRINGING MORE SUFFERING INTO THE WORLD
5. They are probably fluffy and warm
6. Teeth and claws may tear your skin, but a child's words will hurt you forever
7. They won't ever become smarter than you and start correcting you on things
Probably a lot more as well that I can't really think of right now.