Antoine has a casual sexual relationship with the owner of his favorite bar. Her name is Francoise, and like Antoine, she's more than happy to have sex with no strings attached. It just goes to show you, though, how alienated Antoine's relationships with other people are. For him, sex isn't an expression of love, but just a way of killing time. He's not exactly the romantic type.
She takes pleasure in it (she has to have a man a day and she has many more besides me) and thus I purge myself of a certain nostalgia the cause of which I know too well. (3.9)
Antoine knows the score. He's not the only male lover whom Francoise has, and he doesn't seem to care. When he says that sex helps him to "purge" him of a certain "nostalgia," he is probably talking about his memories of being with Anny, whom he truly loves. Without Anny in his life, the most human contact Antoine can hope for is meaningless sex.
He can't make love any more? But he has made love in the past. Having made love is much better than still making it. (16.95)
For Antoine, the memory of having sex is always better than the act of having sex. When you remember sex, you can look at it through rose-tinted glasses. Or in other words, you remember it as if it was happening in a movie. When you're actually having it, though, it might not be all that great. It might even be—gasp—boring.
The criminal has fled. The child was raped. They found her body, the fingers clawing at the mud. (22.39)
In case we didn't get the point, Antoine wants us to know that sex isn't a beautiful thing that gives meaning to life. More often than not, it's a terrible thing that's forced on people. People who disagree with his negative view of human life might point to sex and love as good reasons to live, but he seems intent on showing us that this definitely isn't the case.
Antoine Roquentin is not dead, I'm fainting: he says he would like to faint, he runs, he runs like a ferret, "from behind" from behind from behind, little Lucienne assaulted from behind, violated by existence from behind. (22.39)
Most of the examples of sex in this book are negative ones. Antoine has meaningless sex with a woman who doesn't love him, the Self-Taught Man makes advances on young boys, and Antoine reads a story in the newspaper about a little girl being raped and murdered. He compares her rape to the way he feels every day, because he basically thinks that existence itself (or the pain of being alive) is constantly violating him.
They're going to sleep together. They know it. Each one knows that the other knows it. But since they are young, chaste and decent […] several times a week they go to dances and restaurants. (24.133)
It's not good enough to keep having sex. The only way the young people will successfully distract themselves from the emptiness of life is to go about the whole "performance" of dating. They'll go dancing, they'll flirt, and they'll pretend that they're after more than just sex. But the fact remains for Antoine that all the kids are trying to do is avoid the crushing feeling of boredom.
Once they have slept together they will have to find something else to veil the enormous absurdity of their existence. (24.134)
When he sees a young couple flirting near him, Antoine knows that they are going to have sex. But he also knows that once the sex is over, the young people won't have anything left to distract them from the absurdity of their lives. Their only options will be to keep having sex or to find a different way of distracting themselves.
He was so little guilty: his humble, contemplative love for young boys is hardly sensuality—rather a form of humanity. (32.1).
There's no getting around it: The Self-Taught Man is sexually attracted to young boys. Antoine doesn't seem especially judgmental about this, because he thinks that the STM's boy-love is a natural extension of his humanism. In other words, the guy claims to love all of humanity equally, and this extends to the realm of sexuality.
A brown hairy object approached it, hesitant. It was a thick finger, yellowed by tobacco; inside [the boy's] hand it had all the grossness of a male sex organ. (32.23)
Antoine knows that something sexual is happening when the Self-Taught Man reaches out to stroke the hand of a boy sitting next to him. He even compares the STM's finger to a gross penis while it's happening. In this description, you can tell that Antoine is more disgusted by the STM than he's letting on.
We have courts in France for people like you. So you were studying, so you were getting culture! (32.25)
When the librarian realizes what the Self-Taught Man is doing with the boy beside him, he (the librarian) yells for the STM to leave the library and never come back. Shortly after, he punches the STM in the face and bloodies his nose. In case the STM didn't realize: making sexual advances on high school boys isn't okay.