Because of one line Bobby said in the first issue. Regardless of how Stan felt about writing a gay character, I find it really hard he intended it this way because of the scrutiny on the comics since 1954.
Is it possible he intended it? Yes. Is it likely? No.
"Seduction of the Innocent is a book by German-born American psychiatrist Fredric Wertham, published in 1954, that warned that comic books were a negative form of popular literature and a serious cause of juvenile delinquency.
The book was taken seriously at the time in the United States, and was a minor bestseller that created alarm in American parents and galvanized them to campaign for censorship. At the same time, a U.S. Congressional inquiry was launched into the comic book industry. Subsequent to the publication of Seduction of the Innocent, the Comics Code Authority was voluntarily established by publishers to self-censor their titles. In the decades since the book's publication, Wertham's research has been disputed by scholars."
source:
https://en.wikipedia.org/wiki/Seduction_of_the_Innocent
--
EDIT: Before anyone jumps on me, some posts ago, I've already written that, nowadays, you can interpret it differently then what Stan *probably* intended to write. And I'm *not* suggesting they should retcon Bobby back into the closet either. I'm merely saying that I don't think Stan intended any subtext back then. That's all.