I think we're at an impasse until we get further developments. I honestly thinking her being bi would make the most sense in the story and there are interactions that I felt like she has begun to develop feelings for both MC and Ai. (I don't have issues with polyamory in principle so multiple people in a relationship isn't an issue for me.) For instance, I think most of chapter 16 can be read as a double entendre in which both Makato and Yukiko are referring to both one another and Ai when talking about "the people they love." (That specific wording yet with very inexact diction I think is intentional on the author's part as it leads to the sense that there is no clear antecedent and there is some ambiguity on both ends, as if either of them meant Ai and only Ai, they would have said her by name.) Earlier, she was also unhappy when she heard Makato was going out with Ai, which you would have thought she had resigned herself to if it was only Ai she was in love with because she knows she only likes boys, but if it's jealousy over a more recent attachment she's grown that she didn't have the time to get over, then it makes sense why she would be jealous and possibly feel excluded.
You're free to disagree with my analysis, naturally, and we still have events to come so the only person who knows how the story will develop for certain is the mangaka, but I'm not just shipping characters for the sake of shipping them, but I have textual and structural reasons for why I would want characters to end up together. If Ai is not honey trap AI, then I think the three of them should all enter a relationship because of how much it is established they all have feelings and bonds for one another and how much they all have sacrificed and endured to get to this point. If she is, then I think MC and FMC would be the second best pairing given the circumstances.
Again, you may disagree with these points, but it's clearly not me trying to live vicariously through the main character or wish-fullfillment, but because I think that these are the best options for what has been set up.
@TitanAnteus
I wouldn't be against that considering that both of them share more in common with one another and have more chemistry than either of them have with Ai. Honestly, Mikazuki and Yukiko even share more chemistry than Yukiko or Makato has with Ai, and those two are the least likely to end up together given she shot him in the head.
Honestly, I like all the characters in the story and they're developed thoroughly enough. I think whilst MC isn't nearly as complex as he could be, he's still a good character and works as a competent straight-man to the events around him and does show distinct characteristics unique to him.
@tokuzawa_keisuke
I probably would be less of an obvious hint to a non-native English speaker. In Japanese, it's probably subtle wordplay, like when someone names a character based off of Latin or Greek words in English.
@justforthelulz