In particular, the concept put forth by "queer theorists" (for lack of a better term) that homosexuality is a social construct and that before the 19th century and the "medicalization of sexuality", no one was really straight, gay, trans, etc.

EDIT: I found out that user sowser answered a similar question a short time before I posted this, but if anyone else has something to say about the issue I'd be glad to hear it.