[Sylvie] had no idea where babies came from, even on her wedding night she remained baffled. (3.12)
Sex education isn't a thing in these days, and it seems like some people are taken by surprise on their wedding night. What do you think of this?
Yet again, Sylvie claimed to be taken by surprise by the newest addition to the family. […] Once, Sylvie had had no idea how children were started, now she seemed uncertain as to how you might stop them. (20.29)
While sex was surprising for Sylvie as a young woman, now she can't seem to shake it. Although, her fidelity is questionable, so who knows who her kids belong to at times.
Ursula shuddered at the memory of Howie's "parts" and how they had come together with hers in one vile conjugation. Was this what Sylvie and Hugh did? She couldn't imagine her mother putting up with such a thing. (20.287)
When Howie rapes Ursula on the stairs, it affects her entire view of sexuality going forward. She wonders if all sex is like this, and her mother's puritan attitude toward the act only reinforces the belief that sex is bad.
"My life would be so dull without your salacious reports from the front line. What a deal of vicarious excitement I derive from your love life—or whatever you want to call it." (21.57)
Pamela and Ursula are less prude than their mother, and they have no problem talking about Ursula's love life. In fact, Pammy relishes it, being in a boring ol' monogamous relationship of her own.
The war made indiscretions easier. (21.56)
When people are dying all around you, a little extra sex isn't going to hurt anyone. People have no idea when they're going to die, so they have to enjoy life while it lasts.
Ursula was still a virgin, of course, "intact" as Sylvie would have it. Not for any moral reason, simply because she hadn't yet met anyone that she liked enough. "You don't have to like them," Klara laughed. (24.57)
In this timeline, Ursula is a little more sexually reserved, and this quote also highlights the difference between Ursula's prim English upbringing and the Germans' different view of sex.
(Did Eva call [Hitler] mein Führer even in bed? It seemed perfectly possible.) (24.101)
Yep, thinking about Hitler's sex life is perfectly natural in this book.
"[The war] seems to be everyone's excuse for bad behavior," Ursula grumbled. "If people believed in eternal damnation they might not be seizing the day quite so much." (25.590)
Ursula seems to have a different opinion of sex as she gets older, which we see in this scene. What changes her attitude in the days since she was having an affair with Crighton, and sleeping with both Ralph and Fred Smith around the same time?
"[Jimmy's] a homosexual, you do know that, don't you?" Pamela said. […] It was information, not censure, but there was still a mild prurience in her words and the faintest trace of smugness, as if she were better able to cope with liberal views. Ursula wondered if she knew about Gerald and his "proclivities."
"Jimmy's just Jimmy," [Ursula] said. (25.617-25.618)
We're not sure why these details are included about Jimmy and Pammy's son, Gerald, but there they are. Pammy, despite being "liberal," defines Jimmy as a homosexual, but Ursula sees him as simply her brother, no matter what his "proclivities" are.
[Benjamin] went into a kind of spasm that [Ursula] thought might be a prelude to death by apoplexy. […] "Sorry," he said. "Didn't mean to do that." (But what had he done?) (26.172)
Premature ejaculation probably isn't discussed in Dr. Beatrice Webb's sex-ed book that Ursula reads, but that's what happens here when Benjamin experiences a premature "little death," as the Victorians so lovingly called orgasms.