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. The Earth's leaders have drawn a line in the interstellar sand—despite the fact that the fierce alien enemy that they would oppose is inscrutable, unconquerable, and very far away. A reluctant conscript drafted into an elite Military unit, Private William Mandella has been propelled through space and time to fight in the distant thousand-year conflict; to perform his duties without rancor and even rise up through military ranks. Pvt. Mandella is willing to do whatever it takes to survive the ordeal and return home. But "home" may be even more terrifying than battle, because, thanks to the time dilation caused by space travel, Mandella is aging months while the Earth he left behind is aging centuries.