Separating Gender and Sex
A woman doesn't have to be born with a vulva to be a woman, a man doesn't have to be born with a penis to be a man. While not all cis people believe this, all trans people know it to be true. Yet too many trans people and cis people who believe that still expect man and penis and woman and vulva to be related. Too many cis people feel that it's acceptable to refuse to use the correct terms or pronouns for a trans person until they have "the op", SRS. Too many trans people don't consider a trans person who isn't going to get SRS to be "really" their gender.
Cis people are not the gold standard that people of their gender must be measured against. A trans woman is just as much a woman as a cis woman is. Full stop, regardless of what kind of woman she is her womanhood is not open to debate. Even if she's a butch tomboy who doesn't mind getting dirty and isn't all that comfortable in frilly dresses. And this goes farther, it means that she is a woman even if she's perfectly happy with her penis, even if she doesn't want to start on hormones. A cis woman's body isn't the only kind of body a woman can have.
Actually, that's not even true because there's no such thing as "a cis woman's body". That makes it sound as if there's only one body type among cis women, which just isn't the case. Cis women come in all shapes and sizes. Some have breasts so large that it causes them pain, some have chests so flat that they'd never really have to wear a bra. Some have higher levels of testosterone and can easily be read as cis men if they wanted (and even if they don't want to). There are cis women who are so short that they're easily mistaken as children, and others who tower over most cis men.
The same goes for cis men, there's just as much variation among them. Some naturally have large breasts, some can easily be read as cis women if they want (and even if if they don't want), some are just as short.
It only makes sense that there'd be as much variation in the kind of body that trans people would be comfortable in. But they're expected to want the "ideal" body of their gender, whether it's what they really want or not. A trans man can never be tall enough or strong enough, a trans woman can never be dainty enough or have large enough breasts.
The idea that a person has to be trans to want the medical treatment involved in transition is problematic as well. It goes to say that there's an "ideal" body for each gender. A woman can't want to have smaller breasts, and certainly can't want a flat chest, or facial hair, she certainly can't want to have a penis. A man can't want to have breasts or not to have body hair, he certainly can't want to have a vulva. That may be true for most men and women, but the idea that it can't be true for any man or woman? All that does is say that the ones it is true for aren't really men or women. It allows societal gender roles to have more control over our bodies than we do.
If a man wants a penis and a flat chest, he should be allowed to have them. But if a woman wants them, she should be allowed to have them as well. If a woman wants a vulva and breasts, she should be allowed to have them. But if a man wants them, he should be allowed to have them as well. Gender is no more defined by the body you have or want or need than it is by the body you're born with. The only thing that defines gender is who the person is inside.