"Straight" also means "candid." Until quite recently, most gay people were careful to not be candid about their sexuality. It also means "honorable." In the recent past (although not the distant past, such as ancient Greece), homosexuality was considered dishonorable. It also means "reliable." I remember the few gay kids in my high school (in the late 1950s) never attending the school dances. This might have been interpreted as unreliability, especially by the people who didn't know.