It's strange how Arata fixates on the hypothetical person you'd be in love with being a girl. I don't get why he's limiting options like that. If people could only fall in love with girls, guys would not be loved at all. I don't think that's how it works.
Then there's the massive implication that being unmarried has to be a problem when other people in your friend group aren't. Like, you don't need to conform to what other people are doing. If they're your friends they won't hold it against you if you don't marry someone, even if they did marry someone.
As per usual, loads of weird assumptions/messages, but it all works out, because of course it does. Ohga is attracted to girls, specifically Kariu. He's not gay, bisexual, aromantic or just downright not interested in Kariu. He's just dense. Boring.