"Carnival," by Elizabeth Bear

Carnival's plot is wildly and enjoyably convoluted, but I'll try to convey the gist: Michelangelo and Vincent are two diplomats (and former lovers) from the Old Earth Coalition Cabinet who seek an audience with the Parliament of New Amazonia, a planet that's managed to remain independent. On the surface, the two men are supposed to broker an alliance. Secretly, they've been sent to discover the source of New Amazonia's miraculously clean energy and claim it for the Coalition. Unfortunately for them, the powers that be in New Amazonia have their own ideas.

Old Earth and its allied planets are under constant pressure from the Governors - a manmade race of artificially intelligent beings programmed to maintain ecological balance. Part of that balance includes population control and resource distribution - the Governors will go into Massacre Mode and start culling if a planet's population gets too high or if the environment starts to decline. The tyranny of the Governors has forced the Coalition to adopt a desperately expansionist philosophy: the more planets they colonize, the more space and resources there'll be to go around, which means fewer excuses for the Governors to intervene. On Coalition planets, resources are tightly controlled, and neither mediocrity nor nonconformity are acceptable.

The Coalition's societal structure is twelve kinds of fucked up, but we quickly learn New Amazonia's freer way of life isn't automatically better. In New Amazonia, women are the ruling class while men are second-class citizens categorized into two camps: "stud" males who have to earn their status by performing gladiatorial challenges, and "gentle" (gay) males who are permitted to be servants and artisans.

Carnival is a clever and intricate science fiction novel about the many ways in which different societies fail to live up to their ideals. The novel encourages you (at least at first) to view the Coalition as the soulless aggressor, but New Amazonia has troubles of its own. Both societies feel they have the science and the history to back up how they've structured their worlds, but their vast generalizations leave swaths of people out in the cold.

It helps that all three protagonists (Vincent, Michelango, and New Amazonia's Lesa) teeter on the edge of being outcasts in their respective societies. Vincent and Michelango have to officially hide their homosexuality from their superiors (the inability to reproduce is seen as a waste of resources), while Lesa rages that her soft hearted, intellectual son will soon be forced to give up his studies because of his gender. While all three are strongly influenced by their environments, due to their outcast status, all three are clear-eyed enough to spot the flaws in their ways of life.

Carnival also asks: should one accept one's society's inevitable inadequacies to preserve the peaceful status quo, or should one risk death and sacrifice to form a better society - even if that society will eventually wind up failing somewhere down the road as well?

Carnival isn't perfect (it can be incredibly difficult to keep up with all the double-triple-quadruple crossing and double blinds going on), but it uses unique settings, nuanced characters, and fantastical set-pieces to ask deeper questions of the reader.
A-