Modern, old and ancient art often depicts genitals, and so do the books covering the subjects. Museums and art galleries are also full of examples of them (usually attached to someone) and are often gawped at by people of all ages. They may generate a chuckle or a snigger but they are not looked on as disgusting "should be in a brown envelope" items. I think the joinery image is far too vague.
It only needs to hint at the sexual act. This is not exactly seen as porn and is on display to all in the Tate.
There's a story about early Russian and American joint space enterprises, but it could be just an urban myth.
When they decided between them to dock a US capsule with the Russian Space Station they had a big argument about who should have the 'female' docking adapter and who should have the 'male' one.
What age is this book aimed at?
Anyway, old films used to use clips of trains entering tunnels, or reverse clips of chimneys being demolished.
But if you look the word up it refers to far more than just sex, it can also refer to other social interaction, apparently.
You may understate the hypocrisy.
If my colleagues (and my own eyes) are to be believed in even a small part, one may persuade a Filipina prostitute to do nearly anything at all: the only thing off-limits is to suggest by word or deed that a prostitute is in fact a prostitute. That suggestion would constitute a Grave Oral Defamation, and could lead to both criminal and civil penalties.
I, myself, did not partake.
I don't believe that your cover is discrete enough. Even an academic treatise on the subject could venture into something a little more carnal. How about a picture of a statue? This could be both discrete and explicit enough to attract the eye and convey meaning. Also, I don't think the font colour works very well with the background.
Isn't English a wonderful language? Discrete and discreet: so close together and yet so very far apart!
When I think of metaphors other than a mortise-and-tenon joint, I find them to be more explicit, so I think you've hit a good balance.
I had to look at it twice to see that it wasn't a reference to carpentry, even after wondering if you meant Geniality (which, clearly you don't, though a certain amount of congeniality would be relevant).
An interesting approach to a delicate subject.
It is too abstract.
I have a book full of images similar to that. It is an old carpentry book called Woodwork for Schools
At first glance I thought your book covered the same subject.
I had to look the word up due to the 'ity' and the Miriam Webster dictionary actually asks people where they got the word from!
But from what I can gather it means far more than just sexual intercourse and seems to be a common word in some psychoanalyst circles.