How we cite our quotes: (Chapter.Paragraph)
Quote #1
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.
Quote #2
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.
Quote #3
"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.