Short answers, pulled from the story.
The American Psychological Association defines sexual orientation as an enduring pattern of emotional, romantic, and sexual attractions to men, women, or both sexes. It also refers to a person's sense of identity based on those attractions, related behaviors, and membership in a community of others who share them.
No single controlling cause of sexual orientation has been identified, but scientists favor biological explanations involving genes, prenatal hormones, and brain structure. There is no substantive evidence that parenting, early childhood experiences, or sexual abuse influence sexual orientation, and the evidence for nonsocial biological causes is stronger, especially for males.
The fraternal birth order effect is the finding that a male's probability of being gay increases with each older brother he has from the same mother. The odds rise from about 2% for a first-born son to 4% for the second and 6% for the third, increasing 38 to 48% with each successive male pregnancy, and the effect appears only with biological brothers, not step-brothers or adoptive brothers.
The Kinsey scale, also called the Heterosexual-Homosexual Rating Scale, was first published in Sexual Behavior in the Human Male in 1948 by Alfred Kinsey, Wardell Pomeroy, and Clyde Martin. It rates a person from 0, exclusively heterosexual, to 6, exclusively homosexual, treating human sexuality as a continuum rather than two discrete categories.
The term sexual orientation was introduced by sexologist John Money in place of sexual preference, on the grounds that attraction is not necessarily a matter of free choice. The American Psychological Association states that sexual preference suggests a degree of voluntary choice, and its Committee on Gay and Lesbian Concerns listed the term as wording that advances a heterosexual bias.
Prevalence varies with the component measured, since assessing attraction yields figures two to three times higher than those for same-sex behavior or identity. In Western cultures, surveys find on average that about 93% of men and 87% of women identify as completely heterosexual, while a 1998 report placed statistical findings between 2.8 and 9% for males and 1 to 5% for females in the United States.