I don't have good numbers for birthing without any anesthesia either, but the Australian Institute of Health and Wealthfare claims that "In 2022, 4 in 5 (80%) women who had labour in Australia received pain relief"[2].
Giving birth is a "0-3 times in a lifetime" event, while sex is a much more high-frequency thing. I'm assuming even people who engage in these fetishes do that every time they have sex, but still many more times than they'd give birth.
So let's try to come up with some numbers: lets say we have 8 billion people on earth, and 1% of the adult population engages in watersports or scat, and they do so 10 times a year for 30 years of their life. That's 300 * 80M = 24B times someone eats shit / drinks piss in the next 30 years.
On the other hand, let's say we have ~4 billion women on earth, each gives an average of 2 children in the next 30 years, and 20% do that without pain relief. That's 4B * 2 * 0.2 = 2B births without pain relief in the next 30 years.
I don't think that what you're saying adds up.
Sources:
[0] https://en.wikipedia.org/wiki/Urolagnia
[1] https://en.wikipedia.org/wiki/Coprophilia
[2] https://www.aihw.gov.au/reports/mothers-babies/australias-mo...