Children who experience parental divorce are less likely to marry heterosexually than those growing up in intact families; however, little is known about other childhood factors affecting marital choices. We studied childhood correlates of first marriages (heterosexual since 1970, homosexual since 1989) in a national cohort of 2 million 18-49 year-old Danes. In multivariate analyses, persons born in the capital area were significantly less likely to marry heterosexually, but more likely to marry homosexually, than their rural-born peers. Heterosexual marriage was significantly linked to having young parents, small age differences between parents, stable parental relationships, large sibships, and late birth order. For men, homosexual marriage was associated with having older mothers, divorced parents, absent fathers, and being the youngest child. For women, maternal death during adolescence and being the only or youngest child or the only girl in the family increased the likelihood of homosexual marriage. Our study provides population-based, prospective evidence that childhood family experiences are important determinants of heterosexual and homosexual marriage decisions in adulthood.