RickTheKick

Why do permanent enlarged mammaries exist apart from sexual selection? Short answer they don't otherwise exist, they are a secondary sexual characteristic, that does not exist in any other species. (There may even be actual places in the world where this applies, that without selection the bigger breast never develops - in regions of Africa where breasts are less of a focus of sexual selection, you tend to find the typical cup size is A. Check a map on this.)

However 'breastmilk' is the normal term, to distinguish it from formula. The people who would be upset are so few in number that it makes more sense to have an individual approach for them when they ask for it, than to expect everyone else to change.

That's all very well when it comes to "performing" gender, ie wearing a dress and carrying a purse. When it comes to how society treats you, that usually has to come down to sex, because if it didn't then we could treat men and women the same. Someone can pursue whatever lifestyle they want, without having to justify it to anyone else, and other people can be polite. But when someone can choose based on their identity to compete with biological women in a sport or even in a fight, or choose to enter a women's prison even if they are a male sex offender, that is not about their lifestyle or a social construct. The reasons that society treats women differently are biological sex related.