nah, it’s not based in bloody reality or anything, it’s the Japanese unwillingness to acknowledge that there are women who exclusivly date women aka Lesbians or hell even less mention the idea of Bisexuality!!!
it’s a shity troupe born from homophobia even if the authors writing bloody YURI still use that shity idea!
Though I do agree with you on the part about homophobia and especially the existence/portrayal of bisexuals in manga, I do not think that applies to this particular scene and couple. This manga hasn't shied away from establishing that characters are exclusively into the same gender (Saeki) so I see no reason why the author would do that with this couple, it's also implied that Miyako is exclusively into women.
In this particular case it seems that Riko just has a much stronger preference towards men. Though it obviously differs a lot from person to person and each person's own experience but it's not uncommon at all for someone bisexual to be much more into one gender except for specific cases, be it specific people or a specific look. I can only speak from my own experience but when you do lean heavily in a specific direction, it's easy to just not give it any thought until you come across one exception that makes you reconsider it all.
On that note, it is really nice to see some bi rep even if it is just a side couple.