First off, every ethical argument for having children is dominated by other options that are more effective. 

1) If you’re worried about population issues, just donate $10k to bednets. That’s roughly the equivalent of two extra children existing in the world. 

If you only care about population issues in your own country, then there are undoubtedly charities that save/create more lives in your own country that are cheaper than having your own children.  

I would also recommend questioning your beliefs that people in your own country matter more than other countries.

2) If you’re worried about “idiocracies” and think your genes would make the difference, donate your sperm/eggs, and call it a day.

This also addresses the whole “you owe it to your ancestors” logic.

If you think you’d provide a better environment for the children than other people, remember that having children that feel like a burden is not a good environment for children.

And regretting children is more common than you ever hear about due to cognitive dissonance and the social stigma about not liking parenthood. (Check out the subreddit “regretful parents” to see what I mean).

Also, mentorship and advocacy is way more scalable and effective per effort than trying affect your own kids over decades. 

Just go around promoting good ideas and altruism to young adults and you’ll affect way more people.

3. If you think you “owe it” to your parents, consider how unethical it is for somebody in a position of power to force an uninformed, non-consenting minor to sign a binding, life long contract.

This is what it would mean to think that children, by dint of being born, owe their parents grandchildren.

4. If you think it’s “selfish” to not have children, that’s just incoherent.

Having children then neglecting them is selfish. 

If you don't have children, there’s nobody to be selfish to.

Unless you try to appeal to it making the world a better place to have children, and then you’re back to donating bednets and sperm/eggs, which is the more effective option for making the world better.

You could try to say having children makes you happier, but then that’s a selfish argument to have children, not an ethical one. 

And, also, that's a highly questionable statement. The research is very unclear about this. It probably varies a ton based on the person, the child, the environment, and the counterfactuals. 

In fact, given the opportunity costs of having children, where you could be spending that time and money on more effective ways to help the world, like helping sick children in the developing world, there are much more compelling ethical arguments to not have children.

Except I think it’s a bad idea to make ethical arguments either way about having children.

The drive to have (or not have) children is like the drive to form (or not form) romantic relationships.

Having children is a deep, probably unchangeable, human need, and some people have it, and some people have the opposite.

Saying that it would be unethical to have children would be like trying to say it’s unethical for people to form romantic relationships because it takes time and money away from impact. 

Saying that it’s unethical to not have children would be like trying to say that it’s unethical for asexual people to not form a relationship, because having a relationship makes you happier and it's better for society to have married people. 

Trying to contort your brain into either configuration if you have a strong desire to have kids or not have kids will similarly just make you miserable.

Have children if you really want them.

Don’t have children if you really don’t want them.

If somebody tries to ethically pressure you into either decision, reason with them calmly, then ignore them and carry on with your life.