Library of Congress Subject Headings are classifications agreed upon by all US libraries and can be a helpful tool when you are searching for information. However, LC Subject Headings regarding sexual orientation and gender identity often do not reflect up to date or fully accurate terminology.
Older materials especially will have the subject heading Homosexuality.
Instead of Transgender, the LC Subject Heading is Transgenderism.
Experiment with different keywords. If you have trouble finding what you're looking for, just ask!