Yumi has been flustered at Hitonari, iirc, and had a spot of "oh no, she pretty" upon summoning Lorelei. Tho both cases cleared up quickly.
In my headcanon, Yumi is aesthetically attracted to some people of all genders, demiromantic, has never really stopped to think about being part of anything polyamorous herself despite being grumpy about an unnecessary easy-to-resolve anime love triangle or two, and has more urgent things to do right now than think about pre-, peri-, or post-marital hanky-panky. She's not blind to people being attracted to her or to others, but she has a complete blindspot where Kohaku's feelings are concerned because... well, they've only met up again in the last few days and things have not been conducive to recognizing such feelings. Nor would acting on them before Kohaku's been able to get her feet under her be something Yumi would be okay with.
Heck, if headcanon!Yumi stopped and realized all the romance flags she's been setting up already, she'd probably feel guilty or manipulative until she slaps herself, blinks when that does literally nothing, and that jolt of weirdness resets her brain enough to remind her that she at no point did it intentionally and that every word was genuine in the moment.
Besides, this is strictly a TV-14 LV show! (I'm making that bit up, I have no idea beyond what the TOS say and the very Not For Television goreshot with Oracle. Let's say that part is censored for airing.)
And if all that sounds like a lot, I have a shipment full of salt here for you to pick a grain to take it all with. It's like 430am here, I'm probablly two "seeds" short of an Exeggcute by now.