Robot sex would mostly pull from the masterbation pool.

It seems to me that one could make the same argument for oral contraception. I didn’t go ram the data through statistics software, but I have eyeballed the TFR history of some countries, and you can see a correlation between legal oral contraception being associated with a falloff in fertility (side note, though – the same couldn’t be said of abortion, or at least I couldn’t see it).

There was a talk I remember a while back from Eric Kaufmann, a Canadian professor of politics. Kaufman’s particular talk was focusing less on the overall impact on countries of low fertility and more on the internal political changes – in the presence of oral contraception, the religious tend to have a substantially higher number of kids than do the irreligious, which has very real political impacts:

https://www.youtube.com/watch?v=IYEyv5a_3LM

Secondly the link between sex and procreation has now been broken by contraception. So, how large your family is is increasingly a choice and when it becomes a choice your values – whether you’re secular or fundamentalist – take on a much greater importance and so it’s really as we move into modernity that cultural values start to matter more for fertility and – this comes out of something called second demographic transition theory in demography – so the link there, sorry the trend, towards very low fertility is spearheaded by secular people. Throughout the world, next to a woman’s marital status, and her education, her religiosity is the most important predictor of how many children shall have, and so seculars are spearheading this trend towards very low fertility and the religious – particularly fundamentalists – are resisting this shift, which is in fact the shift that’s driving the great shrinking that we’re talking about, or at least one of the causes of the great shrinking. And not only that, as populations decline, as fertility rates drop, the percentage difference between religious and secular increases. Because if you think about it, if fundamentalists have five children and seculars four, that’s only a 20% advantage. If fundamentalists have two and seculars one, that’s a 100 percent advantage.

So, I’m more interested in the impact of reduced fertility on a country overall than Kaufmann, who is interested in the effects on its internal political makeup, but I think that he’s got a real point that there are some very considerable long-range effects of breaking the link between sex and procreation. I’d expect various forms of simulated or robotic sex to tend to travel further down the path that oral contraception did, create additional downwards pressure on TFR.