I don't think Bobby Marconi is gay. Girls seem to like him. He seems to like the attention that they give him. He also seems to sincerely like Sarah.
I think if you wanted to explore a possibly gay character in Bridge of Sighs, you should focus on Lou C. Lynch. I don't mean to imply that Lou is gay or that he himself identifies as homosexual. Yet it seems like he’s the most applicable character to nominate for a queer reading.
There are many instances that point to Lou’s rather oblique or uncertain sexuality. His gender is derided when other boys call him “Lucy.” They say that he can’t join their club because he’s a “girl.” Of course, Lou isn’t a girl. He wasn’t assigned the “girl” gender at birth, and he doesn't identify as a “girl.” The boys are trying to demean and emasculate him.
A more clearer instance of Lou’s possible homosexuality comes when he and David Entleman kiss. Remember, David and Lou would walk each other home. One night, David kisses Lou on the lips. Lou pushes David away. Yet it’s not clear if Lou does that because he doesn’t want to kiss David or because he thinks his dad might be watching.
I think it’s reasonable to claim that Lou had substantial feelings for David. When David kills himself, Lou is quite distraught. He’s so upset that he feels like he has to conceal his disquiet. It might lead to suspicion.
When David’s dad comes to confront Lou’s dad about their possible relationship, Lou’s dad says, “My Louie ain’t that way.” After David’s dad leaves, Lou’s dad asks him if he’s “that way.” Lou says he’s isn’t. Yet Lou admits to himself, “I didn’t know.”
In Bridge of Sighs, Lou’s sexuality seems rather vulnerable and unformed. It might seem obvious that Lou is gay, but I think it’d be best to let Lou speak for himself. If he says he doesn’t know, he doesn’t know. Maybe he is gay. Maybe it’s more complicated than gay versus straight.