Turning into a person of the opposite sex over night or turning someone's mentality into that of a dog however, is something that happens in our daily lives.
The difference is, the Strange Strap-on is bought in a shop that anyone can go into, whilst with the exception of genitalia, the rest of the biological changes the player character undergoes happen over time, because of accidental exposure to a mutagenic compound that is capable of causing massive genetic changes.
Turning the sister into a dog, mentally, is due to the highly unstable and experimental mind control serum.
See, there's an actual internal logic to those things because they're products of evil application of science, in a setting where it's possible for those things to exist, where currently the collar and strap-on are possible hints of magic (requires removal via ritual/spell), but so far that's all. Hints that currently are only drawn by inference of dialogue with Ash in the sex shop, and has been that way since they were introduced.
Given the way the strap-on works, and has no mention of scrotum & testes with it, then it truly is miraculous that it's capable of impregnation, especially when the player character doesn't undergo any hermaphrodite transformation.