It's a spectacular book. I recommend it utterly without reservation. Not only does it cover sex in space, it also covers eating in space, pooping in space, peeing in space, sweating in space, sleeping in space, exercising in space, fighting in space and working in space. And it is quintessentially Mary Roach. The whole book is basically "let's get over being embarrassed about this and discuss three grown men spending a week in a self-contained environment smaller than a VW Beetle and discuss what that looks like from a human level."
This raises insights that that you wouldn't get otherwise: Americans took forever to put women in space because figuring out how they might pee was too much to handle. Russians put women in space pretty much immediately because they weigh less. Americans have a tradition of a steak breakfast before a launch because it's high protein and therefore low waste. Russians have a tradition of an enema. Americans have a tradition of exhaustive personality profiles and joint training. Russians have a tradition of fistfights. At one point she profiles a mission specialist who explains that he's been training for 20 years on the off-chance that he actually gets to turn a wrench in a space suit for 20 minutes.
Sex actually occupies very little space in the book because, as Roach explores at length, it's a giant taboo that NASA refuses to address. She lists all the stuff that we simply haven't bothered researching because it's so taboo, and then spends a good chapter seeking out a mythical porn shot on the vomit comet (and concludes it never happened). She balances this out with the thunderous amount of effort NASA spends just keeping their magic space toilet working, and exactly why it almost never does.
If you're at all interested I wholeheartedly recommend reading it. It's super-engaging and contains preposterous amounts of little-known, barely-published information about human physiology that actually matters on earth (80% of your poop is e.coli, if you don't shower you reach "maximum funk" about 3 weeks in and plateau off, etc).