Earlier quoted context omitted.
It's not that nothing like this exists in nature, it's that evolutionarily speaking nothing like this existed in the areas human first evolved. Or if they did, we either evolved to resist them, thus no longer see them as toxic, or we're still in an arms race with the life that produces them (which means they would be much less toxic than fentanyl, but still toxic). Plants (and fungi, and animals) evolved to create no…
Could you please link to or expand about plants producing toxins that specifically keep animals from breeding? It's news to me, and a web search has failed me. Is that long-term pest control, or something else?
From wikipedia: "It has been hypothesized that plants use a phytoestrogen as part of their natural defence against the overpopulation of herbivore animals by controlling female fertility.[3][4]"
Though wikipedia also says: "These compounds in plants are an important part of their defense system, mainly against fungi.[19]"
So it could be that plants are principally targeting fungi for the most part, and just serendipitously targeting animals as well.