I think having children when the world is already overflowing with unwanted children is pretty fucking selfish.

Well it is, but it's also hard wired into every living creature. As in, "I want food security and will pursue that at the expense of others" is definitely selfish but it's the expected position.

It's true that we're the only species with the ability to reason about the social cost of this position, but it's also true that every single one of my ancestors has of course been able to fulfil this fundamental contract of life without impediment.

One problem you may not have considered is that raising an adopted child is a very, very different prospect to raising your own child. Usually people wanting to adopt prefer a newborn, because this is more of a "blank slate" if you will, but the vast majority of children available to adopt are not given up by their parents immediately after birth. Many children have been neglected and have on going social and medical problems. Another problem is the prevalence of issues like Fetal Alcohol Syndrome which causes physical and neurodevelopmental problems.