So would you also oppose the idea that a country would try to teach about the effects of having kids early, or how to use contraception effectivly, in order to help avoid the larger social problems that can cause?
The thing is, it's not interfering with people's autonomy to talk about any of those things. It's not a bad thing for young people to think about this stuff. And reproduction does have social consequences. Why shouldn't people be aware of that? Are you saying knowing this makes their choice less free?
There is some very strange purity thinking, or maybe it is almost romantic, around this idea that people have to make their choices to have kids or not in some kind of bubble, untouched by anything other than personal factors.
As far as immigration, there are all kinds of reasons that a very high levels of immigration is not a great solution. It's expensive, it is difficult for a society to integrate that many people successfully without an established and solid community in place, it can create social divisions, and then there are ethical questions about importing large numbers if people from other countries who have been the ones paying for all the social and economic capital required to raise a good citizen.