Actual gender "deception" cases. A recent article by an Israeli law professor identified five cases internationally in which transgender individuals have been prosecuted for failing to disclose their gender history to sexual partners: two cases in the U.S., one in the U.K., and two in Israel. See Aeyal Gross, Gender Outlaws Before the Law: The Courts of the Borderland, 32 Harvard Journal of Law & Gender 165 (2009).
- In the United Kingdom in 1991, Jimmy Saunders was charged with indecent assault on the basis that he concealed the fact that he was born female from two sexual partners. He was convicted, though the Court of Appeals reduced his sentence.
- In Colorado in 1995 - the same state where the Zapata murder took place - Sean O'Neill was charged with false impersonation and sexual assault on the basis that he concealed the fact that he was born female from four sexual partners. He plead guilty to lesser charges.
- In 1997 in Washington state, Christopher Wheatley was charged with sexual assault on the basis that he concealed the fact that he was born female from two sexual partners. He plead guilty.
- In Haifa, Israel in 2003, Hen Alkobi was charged with sexual assault and "impersonation of another person" on the basis that he concealed the fact that he was born female from four sexual partners. He plead guilty to the sexual assault charge, contested the impersonation charge, and was convicted on both charges.
- In 2007, another transgender man in Israel was charged with statutory rape as well as "indecedent acts" on the basis that he fraudulently obtained consent from a sexual partner by concealing the fact that he was born female. (My only source for this case is Gross, who does not give a name and says the case was ongoing as of early this year.)
Notably, all of these cases involved male-identified transgender persons who had sexual relationships with underage women (and in all but one case, with multiple underage partners). It thus appears that it is peculiarly trans men who partner with women who are in danger of being prosecuted for dating without disclosing (whereas women who partner with men are in more danger of being killed by their partners). Additionally, each of these cases apparently could have been prosecuted solely on charges of statutory rape. Thus, these are not cases of individuals being singled out for prosecution based on failure to disclose, but rather cases that would have been prosecuted regardless based on the involvement of minors - though it is clear that prosecutors and/or judges in these cases did have particular reactions of disgust to these gender-variant defendants, and to their involvement of seemingly unwitting young women in what these authorities no doubt regarded as homosexuality.
As far as I can tell, in only one of these cases -- the U.K. case -- did a court rule that failure to disclose one's anatomy or gender history vitiates consent and creates liability for sexual assault. In the other cases, it appears the defendants simply decided to strike a deal, which made sense given that they were probably liable for statutory rape regardless. The latter cases provide no support for the legal theory that these circumstances negate consent. Even in the Saunders case, it's not clear from secondary sources to what extent this aspect of the prosecution was challenged, or even if the assault convictions themselves (rather than just the sentence) were appealed.
U.K. law today. In 2000, the Law Commission of the United Kingdom specifically recommended that the U.K. Sex Offences Act not be interpreted to treat a failure to disclose transgender status as deception that vitiates consent. The Commission reasoned that this would amount to "the creation of a special rule for transsexuals," and accordingly would likely violate the European Convention on Human Rights, which prohibits discrimination against trans people and guarantees the right to expression and recognition of a person's gender identity. (While the Commission stated this recommendation only in terms of individuals who had undergone sex reassignment surgery, it would probably take a different view now that, under the Gender Recognition Act, the U.K. permits change of legal gender in the absence of surgery.)
Regardless, in 2003 the U.K. revised its sexual offenses law, and narrowed the language concerning deception. Following the traditional approach of American courts, the current language covers only situations of deception about "the nature or purpose of the relevant act" or "impersonating a person known personally to the complainant." Thus, regardless of the Saunders case, it appears that UK law today would not treat failure to disclose anatomy or gender history as rape or indecent assault.
Trans people are not liars. Consider why a transgender person would hesitate disclose their sexual anatomy or gender history to an actual or potential sexual partner. Trans people are not sexual predators looking for unsuspecting victims. They live in a society where their gender identity and expression are not accepted as authentic by many, perhaps most people around them - indeed, where that identity and expression are viewed as mere pretense. They live in a society where fear and loathing of gender variance and homosexuality are intense. These fears are linked by failure to understand or accept trans identities, and manifest most strongly as fears of sexual pollution. These facts are particularly salient for trans people who are young, are just beginning or have recently made a gender transition, and who live in rural or socially conservative areas. Add to this the psychic distress, embarassment and extreme shyness many trans people experience regarding aspects of their sexual anatomy and gender history. Being rejected by a partner who does not accept their gender identity is a painful prospect - as is seeking out partners who will desire them precisely because of not accepting their gender identity.
Given this context, disclosure to potential partners is fraught on all sides for many trans people. The partner who will both accept their identity and still desire them will be rare - depending on geography and other factors, perhaps very rare. At the same time, by not disclosing they are not lying: they are simply living their lives and presenting their authentic gender, as they do in other social relationships. And once an interpersonal connection exists, the prospect of coming out can be frightening, and appear very much easier to postpone or avoid.
Trans people are no more liars when they do not disclose these matters to potential partners than they are liars by simply living their lives. This is who they are. Of course, in a long-term relationship, tiptoeing around the personal details will be difficult and probably unhealthy for the relationship. For this reason, and in response to very real fears of violence, trans people usually do disclose to sexual partners, especially when looking for a lasting romantic relationship. While I have little sympathey for other people's anxieties about being sexually polluted by a sexual partner with the "wrong" body parts, it's never desirable to risk distress to others. But I find it difficult to judge morally, and repulsive to punish criminally, the choice of some individuals, particularly in casual encounters or at the beginning of a relationship, not to come out.