The Big Sleep is simultaneously a steamy and a not-so-steamy novel. On the one hand, it's racy because Carmen Sternwood appears completely naked not once, but twice, and the blackmail plot of the novel is centered on Carmen's nude photos.
But on the other hand, there's no sex whatsoever in the whole novel. A few kisses are exchanged, but they don't lead to anything. So even though The Big Sleep might be R-rated for strong scenes of nudity, nothing steamy or sexy ever takes place. Marlowe continually refuses any and all sexual advances and remains chaste for the whole narrative.
Which of course has nothing to do with whether or not he's attracted to the women he meets. He definitely takes note of both Carmen and Vivian's extraordinary beauty. But he's also very dedicated to his job as a detective, and tells both Sternwood sisters that their father didn't pay him to sleep with his daughters. So when Marlowe rejects sex, he does so because it is more important to him to uphold his principles and perform his work honorably and with dignity.