Definitely a good thing. As sad as I am without male parts, I am grateful that I don't have to worry about them being stomped on by overexcited animals.
I'd take it as a compliment.
it means you truly see your friend subconsciously -or instinctually- as a man.
(though I'm a transwoman, and I don't know if there is a possible comparable situation, so my thoughts on the matter might be slightly clouded.)
Can't be doing the dog much good, to land undercarriage first on a hard unyielding human thigh... to be honest, I read the "never mind" realisation as one of "wait, the dog's been to the vet's and it's been done..." then on looking at the caption I saw how I should have read the cartoon. Maybe I've led too conventional a life....
Bit late, but certainly a Good Thing. Forgetting someone is trans and thinking of them as the gender they identify as means you've adapted to them being their new gender. If anyone took offense to that, then I don't know what to tell them.
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Eh, if your friends aren't upset, you shouldn't be! Take each moment in stride, I think.