Poor people do not have more children because of "boredom." They have more children because they are less educated and because of higher infant mortality rates.
Or you know: lack of access to birth control, particularly the pill. A high infant mortality rate doesn't actually explain anything, particularly in developed nations where that's not true.
You're correct that it was a bit of a pontification, but does it not seem like to much of a stretch to say that the more wealth you have the more options you have for entertainment? If you spend a majority of your time stuck at your house around the opposite sex and can't afford to do much else that in turn you will end up having more intercourse?