Now, I’m still not sure whether sex is real or gender is immutable, but I do know that something very cruel is being said here. And I think it has to do with the phrase “who’ll have you.” Let’s do a little close reading and unpacking.
Rowling is clearly addressing some hypothetical person, presumably a transgendered individual, or the transgendered community in the abstract. Her words, on their face, are encouraging: “Dress however you please.” In other words, your attire may not be to my liking (Jo seems to be saying), but suit yourself. “Call yourself whatever you like.” Presumably her hypothetical addressee wants to be thought of as a woman, and accordingly addressed as “she”; clearly, neither Jo herself nor any right-thinking person would see things that way, and will continue to speak the truth. “Sleep with any consenting adult who’ll have you.” Here’s where we get down to it, and I’ll get to that sentence momentarily. “Live your best life in peace and security.” Well, you can’t argue with that. Jo personally has no intention of beating this person with a stick; a backhanded well-wish if there ever was one. Please go on with your existence; it will certainly have nothing to do with my reality.
Now, let’s unpack the key phrase: “Sleep with any consenting adult who’ll have you.” One can imagine a first draft of that line: “Sleep with anyone you like.” But, heaven forbid, someone should take that literally as a license to molest anyone of their choosing—so the disclaimer must be added that the person must be an adult, and a consenting one; a bit redundant, since only an adult may give give their consent to sleeping with you. But Jo clearly does not wish to be misunderstood, given the type of person she is speaking to.
For me, it all really comes down to the phrase “who’ll have you.” And, after all, who would have one such as the likes of you?*
This is something you would say to someone who is loathsome, unattractive, unworthy, and unappealing in every way, or someone you just broke up with. Go ahead, sleap with any consenting adult who’ll have you. Live your best life, you Thing Undeserving of Love.
It’s certainly not “I hope you find the love of your life, and live happily ever after” (presumably the sentiment of the forthcoming fairytale, The Ichabod).