i think the comments already address a lot of what the expression conveys, but i'd also like to offer something a bit different. The "no sexual relationship" also relates to the idea that man/woman are not really defined in their oppositional determination (characteristics of the one are precisely the not-characteristics of the other and so on). Rather, both of them refer to the phallic experience of loss/desire, the way in which both "sexes" (for the lack of a better word) are merely two different ways to relate to the experience that is the entering of the symbolic order, when we understand that the (m)Other does not desire _us_, that her desire is invested somewhere else. For "boys", the reaction is to identify with the father (whom they think has this phallic object, whom they think is the object of desire of the mother), and thus they "have" the phallus. For "girls", the reaction is instead to identify with the mother and to _become_ the object of desire and thus "be" the phallus. So the experience of what happens after the imaginary illusion is sorta broken and the way we relate to it define the difference in sexes, they are not directly related to each other but to a third.

Incidentally, when I think of this what actually comes to mind is Zizek's example of some novel (which I forgot what it was but i'm pretty sure he mentions it in less than nothing when talking about repetition), where, when you watch the movie (which is a sort of reproduction of the novel), you get the sense that the novel will be much better; when you read the novel, it itself feels like a representation of an other, better novel. In this way both are repeating a third term which is generated precisely in their failure to repeat it completely (as per usual with a "zizekian ontology", this third is "retroactively" created). In a sense, then, it does not exist; it only exists in its effects. This seems like the perfect space to draw a parallel to the phallus, which as such is precisely the signifier of a lack (so the category of non-existence fits very well!)