For me personally, I’m not sure I’d bother unless it was a medical necessity. And I definitely don’t want to be one of those much older guys who has the musculature of a 26-year old bodybuilder. I find it unattractive and off-putting, so if it is under my control, I wouldn’t want to look like that or have to live the type of lifestyle required to arrive there and maintain it. On the other hand, I don’t want to be the type of guy who would benefit from it but not get treatment because of shame, lots of that going on, too.
Biggest thing for me, having known a number of cis men getting treatment, is that it really needs to be medically supervised. I know it’s anecdotal, but it seems like side effects are very common, and there are a lot of very serious potential health problems that can occur.
Generally speaking, the terms man and woman are reserved specifically for humans. I couldn’t tell you why, but I suppose it doesn’t really matter.
For pets, the use of boy/girl probably does have a lot to do with how people tend to infantilize their companion animals.
Additionally, the boy / girl terminology is often generalized to cover all animals, particularly when adults are interacting with children and by extension when children are interacting with each other. It’s not uncommon to have a child ask something like “is that a boy rabbit or a girl rabbit?” but it is a little unusual to hear an adult ask another adult that same question, unless it’s sort of tongue-in-cheek or maybe in the presence of kids.