I think there might be some anthropomorphism affecting the opinion, and I know a lot of people dislike "raising someone else’s children ", especially unknowingly.

In humans, this mostly happens to men just due to biology, but I’m sure some women have experienced swapped kids.

It’s not like those animals are betraying defined social constructs while knowing better, so i agree with you more on this, but that would be my best guess for someone reacting so emotionally to it.

(Also there’s far worse things to use for the “no god” argument, like cancer and numerous other vicious diseases or syndromes)