This is an interesting point.

You seem to be implying that rising incomes are causing people to choose to have fewer children. However, it could just as easily be that having children is a drag on people's incomes.

It's probably too much of a broad trend for that to be the case. As entire countries get wealthier, their birth rates drop: it's true in the US, Japan, Germany, Sweden, wherever. If you see a GDP per capita line going up, you will also see a birth rate line going down in nearly all cases. I suspect it's more likely something else: for example, access to and usage of birth control, access to other family planning controls, abortion, etc.

Edit: or even just access to entertainment that isn't "having sex." I remember a conversation with a (very) old woman who'd had many, many children; she said that... well, there wasn't much else to do back then...

And the old woman did not have access to IUDs.

Nowadays, you can have sex with zero risk of pregnancy.

No, no, no. There's a low risk. The risk is not "zero".

The point is it is so low, that combined with access to abortion, sex and having children are effectively decoupled.

Yet that is not what was stated. The risk of pregnancy is not zero. If we really want to get into it, abortions are not 100% effective either. So yeah, extremely unlikely, but still a possibility that one has a child.

You are technically correct, the best kind of correct.

The context of the conversation, however, was that people had children as a non intentional byproduct of having sex, which presumably people had more of due to lack of other entertainment options.

To which I thought it was noteworthy to point out that children are effectively no longer a byproduct of just having sex, especially for the demographics in which birthrates are declining. Hence, regardless of the amount of sex people are having, I would expect the birthrate to still decline.

That does make sense. It's just dangerous to say the chance of getting pregnant is zero, since that could influence someone's decision/understanding and lead to an undesired shock of an outcome.

Maybe the people with the high incomes live in areas where the cost of having children is higher? You often hear about people making 200k at Google on here sharing a two bedroom apartment with a roommate. Personally lack of a additional bedroom in my home prevents me from having another child, and home prices prevent me from moving.

Nobody at Google making $200k has to share a two-bedroom apartment, even in the Bay Area. I was easily able to afford a $2000/month 1-bedroom in Mountain View in 2014 on a salary around $110k, and the same apartment today has only increased in price by 30%, whereas starting salaries have increased much more.

Is it more economical to share a 2-bedroom apartment when you're starting out? Of course, and many new grads share apartments both for money and, just as importantly, social benefits in a place where you might not know anyone else.

No. Declines in fertility are correlated with increasing incomes. Broadly (not uniformly), in all cultures through out time.

It's a thing, look it up.

Definitely true. But the causation may be backwards. Perhaps people are making the decision not to have kids based on how expensive it would be, the expense in opportunity cost and career advancement is higher with the more income you have.