I think your response has been pretty fair OP.
Sometimes we see something and are just appalled by it, and it's hard to see why others don't see it the same way.
I have heard honey bunny and other derivatives as a term for a partner, but I've also heard it used in a more innocent way for children. I think there are plenty of names that can have that double meaning: sweetheart, babe, darling, sweetie etc.
I'm currently calling DS "monkey pants". I don't actually know how that originated, and I probably wouldn't recommend that for a partner as it'd be a bit weird.
I do think there are some nicknames/descriptors that are strictly partner-only and not suitable for a child. The worst one I can think of is "sexy". I absolutely HATE hearing any parent use that phrase about a child "my sexy little boy" - nothing will persuade me that it's not fucking weird and deeply, deeply inappropriate, but not all will agree with me. That's a whole other thread 😂
I think @BoredZelda nailed it really. I can't really find this T-shirt offensive for the reasons you do OP, but honestly, I'm not a fan at all of the different messages we put on T-shirts for our boys and girls.