
No offense, but he rejected Shinoa at least four times now.
He just doesn't like her.
It's not even a matter of ship but of consistency.
He rejected her because he doesn't have attraction to her or girls.
Multiple panels show his disinterest in girl, the "breaking" point arrived when he saw Shinoa and Mitsuba naked and literally didn't care, to which Shinoa questioned it.
Even if he doesn't like neither of them he should still get embarrassed/flustered realistically speaking.
Look away for respect.
That literally didn't do anything for him, he was neutral, straight up didn't care.
Once again, at this point it's not even a point about the "best ship to be Canon", it's just about consistency.
He refused Shinoa so much there's no way he's going to suddenly go and date her, even if he doesn't actually end up dating Mika it's Canon their feelings are mutual.
He rejected Shinoa too much for him to suddenly decide to date her at the end of it.
If Mika dies and Yuu doesn't take his own life he's probably just going to stay single and if not single he will just date another man because the Manga made it clear at this point that he straight up doesn't like girls in general.
Their body doesn't attract him or cause him pleasure.
If he does end up dating a woman it won't be Shinoa though and it will be to "repopulate" since that was a thing mentioned in the first chapters, they heavily push hetero couples with the hope of having earth full of humans again.
But at this point no matter how much they try and repopulate earth is doomed even if they try and save it.
There's not enough humans to repopulate and it could end up with inbreeding and by default extinction, it won't though because as we all know inbreeding doesn't give healthy children and they know it too.
Aside from being disgusting it's also useless because they will either die and if they manage to reach adulthood there's a chance they can't actually procreate and even if they do it's guaranteed the next kid will die as a child or be unable to get pregnant/impregnate someone.
Either way, Yuu rejected Shinoa too much, at this point them getting together is literally something the creator pulled out of their ass to give some "closure" to something that was dead years ago as Yuu rejected her at least four time.
It can't be any clearer than this when you compare how far he'll go for Mika.

He won't, but Mika told him to NOT do a lot of things and Yuu ignored each and one of them because his priority is being with Mika to a point he doesn't care if Mika tells him to not do something, he'll do it as long as he can stay with him.
Even if he doesn't kill himself for Mika and him to be together he rejected her four times, this ship is as dead as Akane and the other kids.
Believing it will happen (here I'm speaking of consistency not about which ship is better, this manga is going on for so long I'm not even trying to follow up this mess of a Lore, I'm simply waiting to see how it ends) is just as delusional as Shinoa still hoping Yuu will chose her after the fourth rejection.
i know what’s going on, and from what i’m seeing i’m guessing mika is gonna end up dying either way and yuu’s friends will come back, everyone will live and guren and his friends will die together, vampires also will be gone and yuu and shinoa will get together. that’s my guess on it. even if it’s my guess i don’t like it