1) Did you know Zhu Bajie (豬八戒, “Pig of Eight Prohibitions”, a.k.a., “Pigsy”), the lecherous swine spirit, was a later addition to the JTTW story cycle? He does not appear in the 13th-century precursor of the novel, while a variant of Sha Wujing (沙悟淨), the complacent water spirit, appears in said precursor and even in Xuanzang’s historical biography from the 7th-century (this is even before the development of Monkey!). But all three of Tripitaka’s demonic disciples appear in a late Yuan-early Ming dynasty (14 to 15th-century) operatic stage play (zaju, 雜劇) by Yang Jingxian (杨景賢) titled Journey to the West (Xiyouji, 西遊記). Regarding Pigsy (fig. 1), acts thirteen to sixteen of the twenty-four act play describe him taking human form, tricking a woman into marrying him, and later kidnapping her, forcing Monkey to take her place in order to defeat the monster (readers will surely recognize this as being identical to Zhu Bajie’s early adventures in chapters eighteen and nineteen of the original novel) (Dudbridge, 1970, pp. 197-198; Ning, 1986, pp. 69-78 & 151-157). Such a complex tale no doubt took time to develop before it was included in the play, and since it doesn’t appear in the 13th-century precursor, I suggest Pigsy’s addition to the story cycle most likely took place during the 14th-century.
2) Did you know the aforementioned play depicts Sun Wukong as a lustful monster? Act nine describes him kidnapping the princess of the Golden Cauldron Kingdom (Jinding Guo, 金鼎國) to be his wife (compare this with Pigsy kidnapping his wife as mentioned above). She is, however, freed by Heavenly King Li Jing (李天王), and the Bodhisattva Guanyin (觀音) eventually traps Monkey under Flower Fruit Mountain (Dudbridge, 1970, p. 195; Ning, 1986, pp. 63-66 & 145-146). In act seventeen, the four monks are leapt upon by lasciviousness maidens in the Country of Women (女國). Tripitaka resists, while Pigsy and Sandy succeed in bedding their respective partners. Monkey tries but is unfortunately stopped by his golden headband.
My lustful nature was about to be aroused, when suddenly the golden hoop on my head constricted, and the joints and bones up and down my whole body began to ache. The throbbing reminded me of a bunch of vegetables. My head hurt so my hair stood up like radish-tops, my face turned as green as smart-weed sprouts, my sweat beaded up like the moister on an egg-plant soaked with sauce, and my cock fell as limp as a soft, salted cucumber. When she saw me looking for all the world like chives sizzling in hot oil, she came around, suppressed her itch and set me free (Ning, 1986, p. 90; see also Dudbridge, 1970, p. 198).
Act nineteen sees Monkey resort to seduction in an attempt to gain access to Princess Iron Fan’s magical weapon (fig. 2). Upon meeting her, Monkey recites a poem chocked full of saucy innuendo: “The disciple’s not too shallow / the woman’s not too deep. / You and I, let’s each put forth an item, / and make a little demon” (Ning, 1986, p. 141). The princess, however, proves immune to his advances, and after an exchange of heated words, she brandishes a sword against him. This is when Sun threatens to rape her: “You Hussy! If I should lay my hands on you, I won’t beat you or scold you, just guess what I’ll do!” (Ning, 1986, pp. 141-142). Ning (1986) ties Monkey’s lustful nature in the play to longstanding Chinese myths involving ape spirits abducting and raping human woman (pp. 143-145). 
1) For the evolution of Sha Wujing, see Dudbridge, 1970, pp. 18-21.
2) See also Wu (1987) for descriptions of said ape tales.
Dudbridge, G. (1970). The Hsi-yu chi: A study of antecedents to the sixteenth-century Chinese novel. Cambridge: Cambridge Univ. Press.
Ning, C. Y. (1986). Comic elements in the Xiyouji zaju. (Doctoral dissertation). Available from ProQuest Dissertations and Theses database. (UMI No. 8612591)
Wu, H. (1987). The earliest pictorial representations of ape tales: An interdisciplinary study of early Chinese narrative art and literature. T’oung Pao LXXIII, pp. 86-112.