You can’t argue too much with the premise of Martin Amis’ new novel: “Sexual intercourse, I should point out, has two unique characteristics. It is indescribable. And it peoples the world. We shouldn’t find it surprising, then, that it is much on everyone’s mind.”

True enough, but is anyone obsessed enough to follow patiently the tiresome carnal mystery around which Amis has built “The Pregnant Widow”? The novel opens with the promise of putting the shifts in sexual politics into some sort of comic perspective — always a good idea, especially from as sharp a stylist as Amis — but eventually bogs down in repetitive speculation about who will sleep with whom and when and how and why. It’s often well written and sometimes provocative. But in other ways, it’s more juvenile than Amis’ first novel, “The Rachel Papers,” about almost-20-year-old Charles Highway, an Oxford hopeful obsessed with a girl. Somehow, reading about the sexual insecurities of an almost-teenager is less taxing than wading through the self-indulgent, impotent fears of a 50-something man.

But Keith Nearing, the thrice-married, middle-aged hero of “The Pregnant Widow,” was also 20 once, back in 1970 when he spent a summer in an Italian castle. Amis reports slyly on the zeitgeist of that era, which so indelibly shaped Keith and his friends (and probably the 60-year-old Amis):

“The Me Decade wasn’t called the Me Decade until 1976. In the summer of 1970 they were only six months into it; but they could all be pretty sure that the 1970s was going to be a me decade. This was because all decades were now me decades. There has never been anything that could possibly be called a you decade; technically speaking, you decades (back in the feudal night) would have been known as thou decades.”

In 1970, Keith — clever, bookish, an aspiring writer who “occupied that much-disputed territory between five foot six and five foot seven” — has a girlfriend, Lily, but their bond is more suited to siblings than to lovers. A problem, then. Another: Lily’s best friend Scheherazade, a formerly unprepossessing blonde, has blossomed into a stunner and much occupies Keith’s mind.

Sometime during the summer at the castle — which hosts a somewhat confusing cast of transient characters — Keith will experience a sexual trauma, though not in the sense we tend to think of sexual traumas. “He was by any definition an adult, and he consented — he comprehensively consented,” our unnamed narrator assures us. “It was the opposite of torture, yet it twisted. It ruined him for twenty-five years.”

That set-up is intriguing, but the revelation as to who and what unhinged Keith for most of his adult life is a long time coming, and when it arrives it’s somewhat disappointing. How could it not be? What is earth-shattering to a 20-year-old isn’t particularly shocking when you’re 50, although present-day segments of the book indicate that Keith still frets about the past. (His coy references to the “sinister refinement,” as he calls the highlight of the life-changing moment, grow tedious fast, especially since the sex act — once you realize what he’s talking about — isn’t particularly sinister.)

Amis is a terrific comic writer, and many of his observations are amusingly relevant. As he reads the great romantic works of English literature all summer, Keith realizes halfway through Jane Austen’s “Emma” that the rules of love have altered considerably. Emma takes an awfully long time to realize that Mr. Knightley is her heart’s desire, but “in 1970, you could no longer love subliminally: the conscious mind worked full-time on love or what used to be love.” Graduates of the me decade may think they love you, but they care most about themselves.

Amis’ body of work is of such magnitude that anything he writes deserves investigation. In such masterful novels as “London Fields,” “Time’s Arrow,” “Money” and “The Information,” he has demonstrated he’s one of the world’s best novelists. But “The Pregnant Widow” feels like an aberration, too slight to make an impact. It’s not even as pointed as Amis’ unfairly criticized satire “Yellow Dog.” He’s covering well-saturated territory here. This book is enough to make one think: Badly done, Mr. Amis. Badly done.