Historically, how did people become identified as heterosexual or homosexual?

Explore the meaning of identity politics and the following questions:
Historically, how did people become identified as heterosexual or homosexual? (Note that these specific distinctions were unknown until the latter part of the nineteenth century.)
How have people become pegged, as it were, to their ethnic groups? (Note that St. Augustine in the fourth century CE may have been “black,” (that is, Afro Roman); it would not have occurred to the Romans to mention his “ethnicity.”)
Speculate as to why these distinctions are so important to Americans. What do they say about American culture?