Science

Eh, for the most part yes but really "species" is a social construct and sometimes two members of the same species can't interbreed at all ; see ring species for example, where for a very brief example, a central population of salamanders can interbreed with northern and southern populations but the northern and southern populations can't interbreed with each other, and if the northern population extends south west and the southern population extends north west they can overlap but still not interbreed but also still exchange genes by both breeding with the central population, super funky stuff!

But then you can also have clearly different species that mostly can't produce viable offspring but sometimes can; rarely mule can produce offspring for example.

There are also different species that can regularly and reliably produce viable offspring such as various species of milksnake and kingsnake (see particularly the imperial pueblan milksnake hybrid of pueblan milksnakes and Californian king snakes), between horses and Przewalski's horse (which even have different numbers of chromosomes), between American bison and domestic cattle, and camel hybrids (who readily and regularly backcross to produce 75% dromedaries or 75% bactrians).

Biology is so messy and interesting! I find it fascinating. I don't mean to undermine your point about racists calling different modern humans different species being a bad thing; it might be an interesting biological discussion but racists poison the well so to speak. The utility of camel hybrids and backcrosses would be evidence enough to show a reasonable person why it wouldn't be a bad thing. Alas, racists are dumb.