You have an obligation to disclose your true biological sex to the hypnotee before there is any agreement to play.The discussion, in this Fetlife thread (a Fetlife login is required to follow the link), makes some good points before it starts getting emotional. The first person to object, for example, pointed out that the phrase "true biological sex" is hard to define and may be viewed as prejudicial to transsexual, genderqueer, or gender-fluid people. They're right; it probably would have been better to say "gender identity" instead. Unfortunately, the discourse has been swirling dangerously close to the drain of ad hominem -- one poster, whom I respect and generally think of fondly, has apparently concluded that because that one sentence exists the whole book is unworthy, and actually wished they'd bought a print copy so they could return it as some kind of protest -- and the real issue is in danger of being lost.
That issue, at least to my mind, is why did I put that rule in the list to start with? There were a couple of reasons for it that I think are worth laying out more clearly than I apparently did.
First and foremost, successful, enjoyable hypnosis requires trust between the hypnotist and the hypnotee. Honest, open, two-way communication is important to establishing that trust. If you are going to engage in an erotic activity with someone, it seems fairly obvious to me that both you and they should be honest about gender identity because, whether you believe it should or not, that matters to a lot of people. Respecting your partner's limits is a fundamental principle of ethical hypnotic play.
Another reason, which was also pointed out in the thread, is that when two people engage in erotic hypnosis together they are both choosing to be vulnerable, emotionally and psychologically, during that time. Getting partway through a hot session with someone you think is a particular gender, then realizing they are actually a different gender, can cause some very unpleasant emotional reactions (again, whether you believe it should or not is a whole other discussion). It demolishes trust, and may contribute to emotional problems that require professional help to resolve. Both parties suffer, and neither one had to or even meant to.
I have to admit the thing that bothered me most was the apparent assumption that because I want people to be honest about their sexuality before engaging in an erotic, sexual practice that I must be some kind of bigot. That hurts. If nothing else, the amount of effort I put into writing the book in a way that respects all genders and all orientations should have meant something. And I do think most people got it, or simply disagreed with that rule and went on to enjoy the rest of the book. Those who didn't aren't going to be persuaded by anything I might write anyway.