There could obviously be no valid reason for visiting Palermo at night in those disordered time except for some low love-adventure. (1.52)
When he gets frustrated with his family, Fabrizio leaves the dinner table and heads into town for sex with his mistress. His whole family knows what he's doing, but he's not ashamed. He figures that he's a man with physical needs that his wife can't always satisfy.
This was what the Prince was thinking as the bays trotted down the slope; thoughts in contrast to his real self, caused by anxiety about Tancredi and by the sensual urge which made him turn against the restrictions embodied by the religious houses. (1.60)
Plain and simple, Fabrizio has sex with his mistress as a way of relieving stress. Sex gets his mind off of things he doesn't want to worry about, even though it only does so temporarily. Plus Fabrizio feels like his style is cramped by hanging out with a priest all day, which is understandable.
"I'm sinning, it's true, but I'm sinning so as not to sin worse, to stop this sensual nagging, to tear this thorn out of my flesh and avoid the worst trouble." (1.68)
Fabrizio believes that committing adultery is okay because he would do even worse things if he didn't have sex as an outlet. (Like start punching people?) For him, cheating on his wife is actually a good thing because if he didn't do it, he'd express his frustration and anxiety in even more damaging ways.
Toward dawn, however, the Princess had occasion to make the sign of the Cross. (1.75)
After he gets home from having sex with his mistress, Fabrizio has sex with his wife, though it's not clear why. Maybe it's love. Maybe it's guilt. Maybe it's horniness. All we know is that his religious wife always feels as though she needs to make the sign of the cross and ask forgiveness from God whenever she has sex, even though it's with her husband.
Perched on an islet in the middle of the round basin, modeled by a crude but sensual sculptor, a vigorous smiling Neptune was embracing a willing Amphitrite; her navel, wet with spray and gleaming in the sun, would be the nest, shortly, for hidden kisses and subaqueous shade. (2.53)
While walking through his garden, Fabrizio lingers to stare at a statue of a Greek god and goddess in a lovers' embrace. According to this book, the Sicilians are a very passionate and sensual people, which you can tell by looking at many of their works of art. Lots of naked bodies hanging around.
[But] this time it was not a matter of black stuff, but of milky white skin, and well cut, yes, very well indeed! (2.70)
Prince Fabrizio compares Angelica's skin to a tailor's cloth when he first meets her. And yes, this sounds like something a serial killer might think. But for him, the comparison just makes him think about how nice it would be to get into bed with his future niece-in-law.
"Had you been there, Signorina, we'd have had no need to wait for novices." (2.80)
Tancredi doesn't hold back when it comes to hitting on women. He tells Angelica that he'd like to have sex with her the first time he meets her, and does it in front of both their families. How's that for Sicilian boldness?
"Then he embraced her again; sensual anticipation made them both tremble; the room, the bystanders, seemed very far away." (4.29)
Tancredi and Angelica can barely keep their hands off each other as they wait to get married. But they manage to hold off on sex until their wedding, and they actually come to savor the anticipation of sex so much that they enjoy it even more than sex itself.