I don't know how to explain it better, but I think Deguchi thinking that shows his insecurity.
Bisexual leaning to the straight side more :D
He's about as straight as a rainbow (=・ω・=)
It's becuz Onoda was straight. He even referred to himself as a straight guy who got seduced by Shima (lol) so I think he's more into girls but Shima and Deguchi were exceptions
I don't know if the author meant it or not, but denial of bisexuality is a big thing. Obviously Deguchi is the one who keeps bringing it up. Onoda doesn't outright deny being straight, but he does find Deguchi's obsession with it to be problematic. But his past with Shima is kind of what saves this from gay-for-you territory, which is another problematic trope.
I really like this manga but I always think its weird that Deguchi says Onoda is straight, like he's liked 2 dudes?! Thats not really straight ╮(￣▽￣)╭