To make you happy, there's a panel where bon is smiling googly at shiemi when they were complimenting how she looked in her uniform, but nothing like rin or shima, but more like koneko.
I agree, bon is very serious and I personally relate, just like tardar said, he could be aromantic.
I do ship him with izumo, but I know deep down that bon is too serious to really be with someone. I guess I ship them for their love-hate relationship. He'd probably prefer shiemi over izumo because she I'm is more kind and tender, which he sees as humble and I guess it works in the monk lifestyle he really likes. Yea