When sex is defined legally, it may be defined by any one of several criteria: the XY sex-determination system, the type of gonads, the type of external sexual features, or the person's social identification.[citation needed] Consequently, both transgender and intersex individuals may be legally categorized into confusing gray areas, and could be prohibited from marrying partners of the "opposite" sex or permitted to marry partners of the "same" sex due to legal distinctions.[citation needed] This could result in long-term marriages, as well as recent same-sex marriages, being overturned.[citation needed]

Dr. Alfred Kinsey's 1948 work Sexual Behavior in the Human Male found that "46% of the male population had engaged in both heterosexual and homosexual activities, or "reacted to" persons of both sexes, in the course of their adult lives".[13] The Kinsey Institute has stated that "Kinsey said in both the Male and Female volumes that it was impossible to determine the number of persons who are "homosexual" or "heterosexual". It was only possible to determine behavior at any given time".[13] Kinsey's book, and its companion Sexual Behavior in the Human Female, have received vocal criticism for their findings and methodology.[14][15][16][17] The New York Times called his research "conscientious and comprehensive"[18] and Professor Martin Duberman called it "skillful" and "a monumental endeavor".[19]