It seems this is meant to normally be off, but I don’t see why it can’t simply be able to be toggled in the options menu.

I'm inclined to agree with you; its really annoying deciding to play a Seraph, Demon, or Dragonkin and then not be able to get anyone pregnant unless your partner is your specific race. And the built in "mod editor" for the game is pretty bad. I've been looking around in my free time trying to figure out how to mod it, but no luck so far.

Can anyone make a mod for that sets this as a default

Not sure if it works or not, haven't tested it yet, but if you go to your Strive settings.ini in your appdata/roaming/strive4power2 area you can change no_breed_incompatibility= to true instead of false and try that.

Edit: So yeah, this doesn't seem to work. Still get the same message about no pregnancy and such. Not sure why the option is there if it can't be used. xD