Worlds Apart — Joe Haldeman

in the year 2085, one third of the Earth's population was destroyed in World War IV... in one day. Then a deadly plague struck, bringing insanity and horrible death to anyone over the age of eighteen. Citizen Marianne O'Hara of New New York, one of the satellite Worlds that are the last outposts of humanity's future, has three choices

To seek out her lover, who wanders the savage Earth, attempting to heal Manson cultists who pursue death as a sacrament --

To remain at home, protected by her communal marriage --

Or to journey into deep space, find a new world and help humanity start all over again