The end of the 19th century, and into the 20th, marked a drastic upheaval in the language, social and societal attitudes, and the literal creation of two opposing sexual classifications (heterosexual and homosexual – words invented in the late 1800s). I believe Dorland was a product of the disintegrating "procreative sex is the normal kind" camp.
The topic is epic, but here's a great article providing a brief history and highlights of the evolution of sexuality constructs. The current use of an ever-growing string of letters to codify and describe people by their presumed position on the sexuality spectrum is generally considered progressive and inclusive. I regard it as regressive, since it perpetrates the same two-sided, "them vs. us," divisive concept as the invention of the words "heterosexuality" and "homosexuality" in the late 19th century.
One brief quote from the article: "A recent UK poll found that fewer than half of those aged 18-24 identify as '100% heterosexual.' That isn’t to suggest a majority of those young respondents regularly practise bisexuality or homosexuality; rather it shows that they don’t seem to have the same need for the word “heterosexual” as their 20th-Century forebears."