Possible response:
1) All cultures do not allow such things. I can't see a culture embracing both slavery and interbreeding (actually, now that I've considered that, there should be a check against that).
Romans, Mongols, Spanish all had extensive slavery and nonetheless interbred with everything that moved
2) The technology requires a special scientific focus, like genetics or cybernetics.
So the ascension perk literally represents an entire civilisation going full Harkness magical realm and becoming xenophile degenerates overnight? What kind of civilisation gets together its best scientists to focus the efforts of the sum total of their knowledge in order to permit waifus: but fungus?
3) There is no natural occurrence that allows a gecko-man to mate with a mushroom.
Nor is there an unnatural occurrence that allows a gecko-man to mate with a mushroom. Consider that there is a graphical culture for a saprotophic fungus which takes over the corpses of other species, that kind of species could be given a trait to represent how it is capable of producing hybrids with all other species simply by taking over their bodies. Conversely, you can have a smooth transition from geckos interbreeding with other geckos, to other amphibians, to all other species, instead of a simple catastrophic fertility event wherein the geckos discovered they were galactic sluts overnight. Imo gradual interbreeding is da wey
*EDIT
You can make the same argument about megastructures or genetic engineering perks.
The perks seem to represent the ideas a society is ok with and invested in.
Isn't that what pop ethos is for? Why for example would two fanatic xenophiles living under a fanatic xenophobe government NOT make gecko-fungus babies simply because the ruler, whom they do not like, told them not to profane nature with their horrifying offspring? I imagine it's more a game balance thing but you can bet I'm going to try and mod it so there's maximum hybridification going on. The only downside is I hope they fix the species UI because it's god awful