I was puzzled by the title as well, seeing as Fassbender and Mulligan's actions are pretty shameless right up until the very end. Never mind the desperate sex, what interested me were the polarities Steve McQueen sets up between this brother and sister: The brother is tall, rake-thin and haggard, the sister is short, youthful and glowing. The brother is super fit and can run for miles, the sister is a slob asking if she looks fat. The brother works some incomprehensible corporate job, the sister is a singer. The brother is totally independent, the sister is totally dependent. Both are lonely.
All of which makes me wonder if the film assembles these gender roles, pushes them to extremes, in order to undermine them -- neither ideal can lead to any kind of society. There needs to be a mean.
Fassbender's character has a penchant for blue -- swathed sheets wrap his privates in the opening shot, as if to put his overactive gonads on ice. Mulligan's performance at the bar is bathed in shimmering gold. The colour returns at the end as a washed-out yellow, which permeates Fassbender's final orgy, him pumping away, looking more and more ill with each thrust. This is the only connection, the only warmth, either sibling is capable of.
It's a striking film, with brilliant performances by the leads. I don't agree with the complaints from certain quarters that the characters lack depth. McQueen is quite suggestive about the possible origins of their malaise, but leaves enough ambiguity about their past and future to let his audience make up their own minds.