What, you ask? How can "sex" be a theme in a poem that takes place in a monastery? Very easily, actually! The speaker of "Soliloquy of the Spanish Cloister" accuses his rival, Brother Lawrence, of having lustful thoughts about the women – possibly nuns! – who bathe in the river outside the monastery grounds. He also owns a sexy "French novel" that he plans to show to Brother Lawrence. Even a glance at a particular page of that novel will condemn Brother Lawrence to damnation! That's an awful lot of sex in a poem written from the point of view of someone who has taken a vow of chastity.
The speaker's description of the two women washing their hair emphasizes their physicality, underlining the speaker's unstated lustful desire for them.
The speaker describes his "scrofulous French novel," like the two women, in terms of the book's materiality, emphasizing its "grey paper" and "blunt type," suggesting that the book is intended to satisfy purely physical needs.