And it's a very soft PG-13 at that. The simply fact is The Left Hand of Darkness is as about as sexual explicit as a biology text book. Here's an example of what we're talking about. Fair warning, this is the most sexually explicit passage in the entire novel:
When the individual finds a partner in kemmer, hormonal secretion is further stimulated (most importantly by touch—secretion? scent?) until in one partner either a male or female hormonal dominance is established. The genitals engorge or shrink accordingly, foreplay intensifies, and the partner, triggered by the change, takes on the other sexual role. (7.4)
See what we mean? The only reason we even gave the thing a PG-13 was because we fear the PG crowd would snicker too loudly at the words secretion and engorge.
As for the characters—the question of sex is brought up often, but the actual deed never actually happens. Both Ai and Estraven refrain from sexual intimacy since they're afraid sex would ruin their special friendship. Their love—while emotionally intimate—never crosses physical boundaries.
Oh, and there's a wonderful scene where Gaum tries to have a one-night stand with Estraven and fails epically.
In conclusion, if you've ever been offended by a National Geographic cultural special, go ahead and bump the rating up to—oh, we don't know—a PG-16 (?). For everyone else, PG-13.