Charles Darwin's evolutionist grandfather Erasmus was something of a rake. He had an inordinate fondness for the ladies, and apparently fathered several illegitimate children. Sex loomed large in his private life; it loomed large in his scientific thinking too. His long poems on plant sex were explicit enough to qualify as erotica.
By the time Charles came along, Victorian reticence about sex was in full swing, perhaps (suggests Tim Birkhead) "as a way of controlling the masses, preventing them from dissipating their energies and permitting them to do only what was essential to maintain the workforce." Whatever the reason, Charles was much less inclined to write about sex for a popular audience than was his grandfather, and for this reason his important work on the role of sex in natural selection was not as influential as it might have been. Only in his more technical writing, which he assumed would not be read by the susceptible masses, did he give free expression to his many detailed observations of animal reproduction.
His work on barnacles was particularly important. He was intrigued by the fact that female barnacles had little pockets on their bodies in which they kept multiple "husbands." Part of his problem was convincing his scientific colleagues that these little organisms were males of the same species, rather than parasites. Elsewhere he describes the penis of one species of male barnacle as being "wonderfully developed", lying "coiled up, like a great worm" and when extended between eight and nine times the length of the animal. Heaven forfend that the eyes of a proper lady should fall upon such a description.
Birkhead proposes that it was inhibition that kept Darwin from more fully developing the idea of sperm competition, perhaps delaying progress in this area by many decades. Darwin's daughter Henrietta, who edited his The Descent of Man and Selection in Relation to Sex, was even more prudish, striking out passages in her father's manuscript she thought unduly suggestive. Birkhead says that Henrietta initiated a campaign to eradicate the stinkhorn fungus (Phallus impudicus) from the English countryside, lest it have a bad influence upon its beholders.