For me, using a reference to a sexual position as a title of a book about a respectable worldwide admired nun /is/ "Disgusting or repulsive", which is definition number 3 in http://en.wiktionary.org/wiki/obscene
NB: I am all but a puritan, I like some disgusting things sometime. I don't care how Hitchens title his books. I just deeply dislike bad faith, a more traditional way to describe trollism.
But let me respond about "obscenity":
You call Mother Theresa "respectiable". Hitchens's whole book advances the argument that she is nothing but, and is in fact contemptible. His argument is not germane to your point, so I won't delve into it.
Hitchens, in a video (you can find it on youtube, can't remember the title), argues that laughing at religious authority is important, and that religion should be mock, because mockery is one of the first steps towards overthrowing an oppressor. So the argument that we can't talk about a "respectable nun" in profane and coarse terms, and instead must treat her with ignorant reverance, is poisonous and cuts to the foundation of Hitchens's argument.
"It (the title) is actually a perfect triple entendre: the book is about the politics of missionaries and the position of those who feel they can enlighten by divine warrant and presume to instruct the rest of us not just in morality but in day-to-day conduct. It was a title I couldn’t not use."
[edit: http://www.youtube.com/watch?v=8t4VhQX5Ckk "Mockery of religion is one of the most essential things because to demystify supposedly 'holy text dictated by god' and show that they are man made and what you have to show [is] there internal inconsistencies and absurdities. One of the beginnings of human emancipation is the ability to laugh at authority..."]
"Finally, I note that he describes the title of my book as “obscene,” and complains that it attacks someone who is “elderly.” Would he care to say where the obscenity lies?"
Hitchens doesn't deny that there is a sexual connotation. Regardless, sexual connotation is not synonymous with obscenity. The point is that he invites Leys to discuss whether the title is obscene or not. (He doesn't explicitly deny the claim of obscenity, you'll note.) This discussion would further Hitchens's goal of discussing religious wfigures with less reverence.
p.s. I'm not the one downvoting you, if it matters.
EDIT The meaning of the word "obscenity" is quite clear, just saying something is obscene does not make it so. If you are asserting there was obscene content you should support your assertion and say what the obscenity was.
If you didn't know that, I understand that you might not see the sexual connotations of such a title. But please don't tell me Hitchens didn't know, it would be the most formidable misunderstanding in the history of debating.
Hitchens is eloquent and direct in his criticisms. Leys, in his reply, is vague and does not answer any of Hitchens's criticisms with any specificity.
Leys demurs and says he has no space for a reply. He then devotes one-third of his letter to joking about Hitchens questioning the use of the word "obscene". His response includes grating ad hominem, with a snide remark that Hitchens must not own a dictionary.
In fact, Hitchens is right and it is Leys who must take deeper council with his copy of the OED. Hitchens, if nothing else, genuinely cares about his use of language. (Note this excerpt: "To represent her as a woman defiled with spittle for her deeds or beliefs is—to employ the term strictly for once—quite incredible." Simply delightful.)
Definition of obscenity, from Wikipedia: "An obscenity is any statement or act which strongly offends the prevalent morality of the time, is a profanity, or is otherwise taboo, indecent, abhorrent, or disgusting, or is especially inauspicious."
Hitchens, when he coyly writes: "Finally, I note that he describes the title of my book as 'obscene,' and complains that it attacks someone who is 'elderly.' Would he care to say where the obscenity lies?" Hitchens knows what he is doing. Most people nowadays don't think the phrase "The Missionary Position" is obscene. Slightly tittilating, yes, but obscene, no. Unfortunately, Leys cannot discriminate the subtlety and smugness in Hitchens's small linguistic victory, and responds blithely and indelicately.
Side note: Classic Hitchens is him saying that people who say that racists are discriminatory could not be further from the truth. Because how can people who dismiss an entire race possess the capacity to discriminate?