Still the bouncing had some physiological effect. Ignatius touched the small erection that was pointing downward into the sheet, held it, and lay still trying to decide what to do…. he thought somewhat sadly that after eighteen years with his hobby it had become merely a mechanical physical act stripped of the flights of fancy and invention that he had once been able to bring to it. (2.14)
Again, unsexy sex is funnier than sexy sex. In this case, the sex is doubly unsexy, in that it's masturbation rather than sex, and in that it's sad or diminished. Ignatius is nostalgic for the unsexy sex he once had.
"You two might of settled down and had a nice baby or something."
"Do I believe that such obscenity and filth is coming from the lips of my own mother?" Ignatius bellowed. (2.245-246)
Usually you don't think of settling down to have a baby as obscene, but Ignatius is very sensitive. It's also worth pointing out that he is very much his mother's son. Mrs. Reilly remembers Ignatius's conception as something shocking, if not as actually obscene, so Ignatius may well have picked up his Puritanism from her.
Suddenly Mrs. Reilly remembered the horrible night that she and Mr. Reilly had goen to the Prytania to see Clark Gable and Jean Harlow in Red Dust. In the heat and confusion that had followed their return home, nice Mr. Reilly had tried one of his indirect approaches, and Ignatius was conceived. Poor Mr. Reilly. He had never gone to another movie as long as he lived. (4.94)
This is the closest we get to a description of sex in the novel, and it's precipitated by a film (Red Dust, from 1932). Ignatius was conceived following a movie—and so his love/hate relationship with film perhaps mirrors his father's love/hate relationship with film, in a kind of Oedipal reflection. Ignatius hates the movies and loves their decadent, sensual appeal, just as his father responded to the decadent, sensual appeal and then hated himself in the morning. There's more on this over in the "Symbols" section.
"Myrna's cure-all for everything from fallen arches to depression was sex." (5.193)
It's not entirely clear whether Myrna even likes sex, or whether she just likes the idea of it. She uses it almost like a tool—which means that, like virtually everyone else in the novel, she doesn't seem to see sex as a pleasure.
"You know that ever since I first met you I have directed pointed questions at you in order to clarify your sexual inclinations. My only desire was to aid you in finding your true self-expression and contentment through satisfying natural orgasm." (7.260)
Myrna also uses sex as a kind of verbal aggression—it's her way of bickering. She tells Ignatius she wants him to have satisfying orgasms in the same way that Mrs. Levy tells Mr. Levy that he's ruined his father's legacy, or that Jones tells Lana Lee he's going to report her to the police.
"Your hostility to my lecture is a manifestation of your feelings of failure, non-accomplishment, and mental (?) impotence." (9.85)
Myrna suggests here that Ignatius's mental ineptitude is linked to physical impotence. The thing is, given his pride in his intellect and his relative disinterest in sex, it seems likely that Ignatius would actually be more insulted by a charge of mental impotence than by one of physical incapacity. Tell him he's not sexy and he'd probably just thank you; tell him he's stupid, though, and he'll really get upset.
"You look like a whore in that orange dress. And what's all these sounds you're making like a slut? You look like a drunk nympho passing out in an alley." (9.129)
Lana Lee is displeased with Darlene's outfit for her striptease. Lana has very definite ideas about how pornography should be packaged and presented, as we see in the care she takes with her own naked photographs. There is filth and then there is filth. Lana only wants the high class kind… despite the fact (or because of the fact) that the Night of Joy is not exactly a high class place.
"When we have at last overthrown all exiting governments, the world will enjoy not war but global orgies conducted with the utmost protocol and the most truly international spirit, for these people do transcend simple national differences. Their minds are on one goal; they are truly united; they think as one." (11.104)
Homosexuals are often stereotyped as being particularly focused on sex, and Ignatius presents an extreme version of this stereotype by asserting that gay people only care about one thing—that thing being sex. It's an analogue to the stereotypes that white people in the novel have about black people. Of course, the person who really only cares about sex is Myrna. She really is someone who would like to see global orgies conducted with the utmost protocol.
Once in high school someone had shown him a pornographic photograph, and he had collapsed against a watercooler, injuring his ear. This photograph was far superior. A nude woman was sitting on the edge of a desk next to a globe of the world. The suggested onanism with the piece of chalk intrigued Ignatius. Her face was hidden behind a large book." (11.307)
It's interesting that Ignatius saw a similar photograph in high school since Lana's photograph (with its educational props) is also intended for a high school audience. In the world of Confederacy, are high school kids the only ones interested in pornography, or for that matter, sex? Ignatius is obviously intrigued as an adult—but is this because of the high school memories and the high school setting? It does, in general, feel like adults are too busy bickering and scheming in the novel to pursue more carnal sins.
Ignatius now realized that in addition to the head and ear, an erection had been bothering him ever since he had awakened. It was demanding attention. "Would you mind leaving my booth for a moment while I inspect myself to see whether I've been mishandled? Five minutes should be sufficient." (13.29)
Just another penis joke. A Confederacy of Dunces believes that you can tell a genius by the fact that all the penis jokes are arrayed against him.