That's the thing - it's not up to you, or me. It's a matter of respecting what a person wants to be referred to as. It's the same as respecting a person enough to pronounce their name properly, even if it's difficult/unfamiliar to you, instead of giving them a nickname to make it easier on yourself.
This is not really the best of comparisons. Gender identity IS what you are, whether your outward physical attributes match that identity or not. It's possible to feel and identity as male, for example, even when you have a body that is perceived as female.
Getting surgery to fix something you aren't happy with is perfectly okay in my book - but there is no surgery that will "fix" your gender identity. It can help align your body closer to what your gender identity happens to be, but a woman is just as much a woman even if she doesn't have ovaries, or whichever female bodypart you happen to believe is integral.
They get offended because "it" is a word that has traditionally been used to a.) refer to non-human objects or animals, and b.) has traditionally been part of the dehumanising of people of colour, people of different sexual orientations, and various other oppressed groups. The word "it" has a history in this context, and it's not really a great one.
It's the same reason why calling a black male "boy" is offensive. The term itself isn't offensive - it's all the history and weight that comes attached to it.
Also, if I went through life simply making decisions based on what was offensive to me, I'd be offending other people all the time. I cannot, and will not, be the norm against which I judge other people. I don't have their background, I don't have their identity issues, I don't have their lived experience of racism/sexism/gender discrimination/whatever discriminatory and troublesome thing has led them to present themselves in this particular way. And because I don't, I don't get to pick a gender pronoun or nickname for them that best suits me.
I mean, if someone came along and insisted on refering to you as something other than what you are, wouldn't you be bothered?
Again, this is not really the best comparison, IMHO. Firstly, because vegetarianism is a dietary choice and gender identity is very complex and psychologically anchored matter of selfhood and being allowed to define yourself as a person. Secondly, I'd buy that veggie pizza for that one vegetarian, because if they're going to show up at my party, I'd want them to feel included and have a good time while they're there, and providing them with food they are able/willing to eat seems like the least I could do.