Complex sentence: Historically, clinicians labeled transgender people as heterosexual or homosexual relative to their birth sex, but most transgender people find this offensive and define their sexual orientation relative to self-defined gender identity.
Split into simpler sentences: Historically, clinicians labeled transsexual people as heterosexual or homosexual relative to their sex assigned at birth. Archives of Sexual Behavior Most transsexual people find this offensive, and prefer to define their sexual orientation relative to their gender identity.