Exactly how steamy is this story?
NC-17
Since the Ares is basically a giant space-roaming dormitory, there are plenty of scenes involving midnight rendezvous and zero g clothes removal. Once on Mars, the precedent has been set: sex is on the menu.
We also get the occasional sexual appetite that might seem odd to some readers. For example, Taneev, Ursula, and Marina live in an ambiguous three-way relationship. And at one point, Janet Blyleven even suggests the first hundred colonists compose a panmixia—i.e., "a group where every male mated with every female" (2.4.5). Sounds exhausting to us, but hey, to each their own.
Now, the reason for all this intercourse is more than just to provide the readers with a carnival of the carnal. The first hundred colonists represent a society, and Robinson is interested in all the gears that make a society tick, including sex. Hence the reason for sexual appetites that might seem abnormal because, hey, societies contain different people craving different things. It's also why sex and politics intertwine to the point where it's hard to separate the two. You wouldn't know it by tuning into C-SPAN, but politics are all about the sexy sex.
So Red Mars gets an NC-17 rating, but it's a rating that serves a thematic purpose.