Because Jiro is the most "gay-coded" character. Because Momo and her gets flustered a few times (not in romantic way but still) and because they make for an especially good contrast. There is not much subtext in the manga, no. It's neither queer nor queer-baiting, it's just that the manga itself make it really easy to ship people. And since a lot of the strong dynamics are between characters of the same gender, people ship tons of same sex pairings.
"But the show is a shounen so there is no yuri"
There's not really any contradiction here. Or at least there might be if you consider yuri as a genre, but you can be a shonen and have a female/female pairing (SnK is a shonen and it has a canon female/female pairing for example).