I didn't believe a moan of it.

Nothing is easier for a young woman than to fake an orgasm.
Some are better at it than others.
Whores are the champions.
Running a close second are the frigid.
Then it's the straddlers who are  between the big explosion and utter deadness.
Occasionally those straddlers trigger a nice one, just by hearing themselves going crazy with those oooos and ahhhhs.

But most men are pretty knowledgeable about such things. They have a built-in fake detector. It has to do with some kind of deep sexual instinct, I think. All those vocals and squirms he witnesses aren't connected with  that deep something within a woman that makes her want to cum. It's all surface. And sometimes you gotta tell her something like that. Just lay it straight out: "I didn't believe a moan of it."