I guess that's how Crowley arrived at it.
Crowley would have already been familiar with the unicursal hexagram, or something similar, from as early as 1899. The Golden Dawn 4=7 paper, "Polygrams and Polygons" mentions a figure called the "hexangle", which "
symbolizes the presidency of the Sun and the Moon over the four elements united in and proceeding from Spirit."
It is unclear whether Crowley adopted this symbolism for his own "Hexagram of the Beast", but it is assumed that he did because he substituted it for the Rosy Cross in later versions of
Liber V vel Reguli. It's got that same sort of Solar-Phallus and Lunar-Yoni vibe to it.
However Crowley wasn't initially a big fan of the unicursal hexagram. In a 1916 letter to Frater Achad he describes the figure as "evil" due to the unequal proportions of the lines. It's not possible to draw an equal sided 2D unicursal hexagram. (Or not one capable of being drawn in a circle.) But he seems to have changed his mind a few years later. Although it's interesting to note that the one appearing in the
Book of Thoth looks like an attempt at a 3D hexagram.
If it had appeared in it, where would it have been?
With the Sun and Moon conjoined symbolism in mind, maybe
XI Lust would be my choice.