The Forever War by Joe Haldeman

Pvt. Mandella is the last heterosexual male. All men and woman in 2124 are conditioned by the government to be homosexual. To curb overpopulation, A perpetual war, endless war or forever war is a lasting state of war with no clear conditions that would lead to its conclusion. These wars are usually situations of ongoing tension that may escalate at any moment, similar to the Cold War but light years slow the war but speed up weapons developments who can advance faster.