Complex sentence: The concept of categorizing transsexuals by sexual orientation originated with Magnus Hirschfeld in 1923, the term was used because early investigators thought the main difference between a male transvestite and a male to female transsexual was same-sex attraction.
Split into simpler sentences: The concept of categorizing transsexuals by sexual orientation originated with Magnus Hirschfeld in 1923. This concept arose because early investigators thought the main difference between a male transvestite and a male to female transsexual was same-sex attraction.