In the comments to the last post, about the supposedly semi-pornographic Agnes Owens cover, the multiply wise Brian Busby points out that "Gray is as wonderful a designer as he is a writer [which] makes his comment even more mysterious." And it's a damn good point. Here, for example, are three of Alasdair Gray's own illustratated covers for three of his own books, all of which are much more explicit than the cover he had a go at.
Genitals, pubic hair and breasts surely beat topless children in the pornography stakes. Mysterious.