Sex in Never Let Me Go works much like sex does at Hailsham: we hear that it's happening all the time, but we never actually get many details. At each stage in her life, sex is a big deal for Kathy. When she's a teenager, it's all anyone can talk about. At the Cottages, Kathy worries about her sexual urges. Then when she's a carer, Kathy and Tommy have sex despite the fact that he's recently had surgery and can't be feeling up to snuff.
In each instance, Kathy keeps most of the details to herself. In fact, the most explicit scene is probably the one during sex-ed when Miss Emily gives a birds and bees lesson using two skeletons (to which we say: ew?). So we know sex is important, but that's about all the steaminess you're going to get here.