Anyway, back to the slippery slope ---
If I am right that all the extremely antinatalist and unprecedented changes now occurring are a sort of inchoate effort of people moving as a social organism to try (unsuccessfully so far) to curb dangerous population growth before a crash --- then most of these measures would lessen births, not be pronatalist.
So birth control and abortion and homosexual marriages and seducing little boys and all that would of course tend to reduce population growth. So would incest that results in deformed births, as is common in incest. Coupling with animals, the same, if people start "marrying" their pets, which will surely be next in line.
But what about polygamy? Polygamy, at least Mormon polygamy, results in huge families of offspring. However, I think the general trend is antinatalist. They have to throw the boys away, for one thing: they can't have seven wives for one man if his sons are competing with him for the daughters of his brother. This does not matter demographically if all males who actually do polygamy are fertile because males can always be counted on, normally, to do what they do and beget children on the women. It doesn't matter if there are only 1/7th as many males if those males are fertile. But there is a problem getting enough females --- that's why all polygamy systems always use very young girls and lock them into marriages with relatives early. Polygamy is a bad situation for females and they quickly learn this and if old enough, they try to get out. It's crucial in both Muslim and fundamentalist and pioneer Mormons to lock them in between the ages of 12 and 14, or they lose too many. But that being the case, they HAVE to use incest: all polygamous systems are incestuous for a very obvious reason: not enough females! All nature provides is a one to one ratio, so unless they can kill out the adults of a wagon train and take all the girl children, as the Mormons did once, they have to use nieces, half-sisters, first cousins, etc., and the inbreeding results in a lot of genetic defects and some very strange looking families, very sad. Incest is antinatalist, because incest victims usually can't breed. The restriction in the gene pool by reducing so drastically the number of breeding males of course much increases the incest burden of genetic deformities.
If I'm right, I would expect to see a quick slide down this slippery slope, with all sorts of dire "marriages" starting to occur that are unproductive of normal children: incest, polygamy, animal, as well as homosexual "marriages." More divorces, abortions (especially of female fetuses in many areas of the world: this has a multiplier effect against future births), birth control. The one thing that won't happen is large, healthy families. That only happens when new areas open up, like the large families of colonial America, and there aren't any of those areas at this time.