We make the same sort of assumptions about bodies based on sex, although I would argue that they are more insidious. Culturally it is given that people with female bodies are women and people with male bodies are men. (Remember that, culturally, intersexed people do not exist.) Even among people who accept that trans people are telling the truth and we are who we say we are, we are still described as "men in women’s bodies" or "women in men’s bodies." This language is very problematic, as it still assumes that a female body must belong to a woman and a male body must belong to a man. To see how silly this is, consider a "man in a woman’s body;" just which woman does his body belong to? It’s not a woman’s body; a man inhabits it, it belongs to a man, so it is a man’s body.
This leads to my main point. Bodies belong to their owner or owners, not to society or anyone else. And bodies belong to people; people do not belong to bodies. Thus, a person’s body is theirs to modify and interpret. Even when given the example above, people tend to huff and say, "Well fine, it’s not a woman’s body, but it’s still a female body." To me, this is still problematic. What precisely about a man’s body is female? Certainly not his brain. (Culturally we also have a problem where we don’t consider your brain a part of your body or your biology.) It’s his body, and he’s male, so I’d argue that his body is male as well.
That usually leads to another huff and, "It’s biologically female, and you’re just going to get in trouble if you deny that." This is where things get particularly sticky. There are two problems with this statement. The first is that it is none of anyone’s business what someone’s biological makeup is, except their doctor’s; and thus the insistence of cis people to constantly point that out to trans people is, at best, annoying, and at worst, intrusive.
A lot of cis people have a tendency to forget that biology is a lot more complicated than "male" or "female." First of all, there are those pesky intersexed people again. There are more people than our society likes to admit who have ambiguous genitals or chromosomal anomalies that don’t fit in with our ideas of neat little male and female categories. (A cursory overview of intersex conditions is listed here.)
Secondly, you can’t describe someone’s entire health profile by deciding which sex box they go into. Bodies are more complicated than that. Even amongst cis people, there are biological overlaps. As a personal example, if I took women’s multivitamins, I’d be poisoned. Even though I have a "biologically female" body. How is that? Because we have different nutritional needs than other people based on other factors apart from sex (in my case, a genetic disorder is what causes this particular issue.) So because my nutritional needs are "biologically male," what does that make me?
The second problem with insisting another person’s body is "biologically this or that" is that, by clinging so desperately to the "your body is really female/male" line, you are still privileging the body over the person’s identity. My argument is that we, as human beings, can all interpret our bodies in a way that makes sense with our identities without killing our bodies or getting dread diseases.
We do this already to minor degrees. How often have you heard someone attach sentimental value to a scar? Or point to a birthmark and say, "It’s a heart" or "It’s a kitty face" or some other thing? It would be pretty rude to come down and say, "No, that’s just an abnormal pigmentation of the skin caused by injury or vascular irregularities. Stop being silly." Clearly, that’s what scars and birthmarks are, but the technical explanation of others is not as important as a person’s interpretation of their own body. These marks may have sentimental value, or it may simply be more pleasant to think of a weird mark as a flower or whatnot than as skin damage. It does not stop that person from seeking appropriate medical care if their marks start showing signs of cancer-like activity. So why should anyone else feel compelled to rain on that proverbial parade?
I argue the same principle applies to trans people. I can call my body male to my heart’s content. It doesn’t mean I am going to pretend that it’s no big deal if I get a lump in my breast. (And frankly it’s not your job to make sure that I check.) I don’t need you to tell me that my body is "really female" or "biologically female." I can take care of my body’s individual needs, such as checking my breasts for lumps, without needing to label my body in a way that I don’t wish to label it.
I fell into the trap of labeling my body as somehow essentially female for a long time, and it caused me a lot of mental anguish. The perceived incompatibility of my mind with my body was the source of a lot of discomfort for many years. But once I took a more subjective stance on my body (realizing that it is mine to interpret how I wish), and fixed the incongruity in my own mind, it relieved a major source of stress in my life.
A turning point in my life was researching sexual homology. (Specifically, looking at this web page.) Seeing the origins of particular genital tissues and the similarities between "male" and "female" sex organs made me realize that I could re-interpret my body to be something I am more comfortable with.
Before, I had a lot of sexual difficulties. I couldn’t really touch myself without feeling castrated and wrong. The things I was expecting to be there weren’t there. My genitals were alien and unpleasant to me. But that was a matter of interpretation. I was thinking of them as being somehow essentially female and therefore I was uncomfortable with them. After researching sexual homology, however, I changed the way I thought about my body. In essence I "re-mapped" my associations.
I started with the penis/clitoris. It hadn’t occurred to be before that the two were homologous organs. So the next time I touched my genitals, instead of thinking, "I don’t have a penis, I have some other weird thing instead," I thought, "Okay, so, I can think of this clitoris as a small penis, because they’re similar." And strangely enough, it worked. I was able to become more comfortable with my body based on my new interpretation.
Your first thought may be, "But it’s not really a penis." What difference does it make to you how I interpret my body? As I’ve said, I don’t ignore my health. I realize that my penis doesn’t work the same way as a normal cis man’s penis. But there are plenty of people, cis and trans, with "abnormal" organs and tissues, who take care of their body based on the way that their individual bodies work. Someone with a prosthetic limb may refer to their artificial arm as "my arm," despite the fact that it’s not flesh and blood, and still attend to the health needs unique to their situation. Interpretation need not be delusion, and in fact, I believe that it can be an incredibly positive action to take.
I realize that my particular solution to how I came to be comfortable with my body is not for everyone. However, I do think that because it was such a positive experience for me that it is worth sharing how I came about it.
The only problem I have with it is the lump thing- men need to check as well. Everyone with breast tissue needs to. TRANS MEN need to unless their surgeon scrapped it completely. Pap smears would be a better point.