Indeed 'inserttaila' seems to be the correct spelling, I'll add it to the bugfix patch at some point. Though, it has almost no effect in vanilla as the 'Enjoys Anal' trait is basically favor text.
The ability to impregnate without testicles is considered a matter of preference. Though it may not be biologically accurate, it does not appear to be a deal breaker for most players. Feel free to change your own code or create a mod for it.
impregnationcheck in newsexsystem.gd only filters out dog and horse because those are only found in the sex interaction. People can be impregnated outside of the interaction, so the full check is in globals.gd.