When it comes to the MCxMasami RO, you have to keep in mind their time period, not ours. Plus, this is interactive fiction, its not real.
As for her being a romantic option, you have to read into the subtext when the MC “flirts” with her. It’s not really flirting, its pretty much just teasing. They do it because they love getting a rise out of her. Whether she gets angry, embarrassed, pouts, etc. The MC finds it highly amusing. Especially when they get embarrassed and puff out their cheeks like a cute little squirrel. They note that they find it adorable/cute and that’s the main reason they do it.
My MC’s (I have multiple saves with both male/female ronins that like women) are always the Charming, Protective Pervert, the other stats I change. I’m perverted with all the females that show up, Masami, Toshie, Momoko. Toshie is too dense to notice your remarks, or if she does she rolls her eyes, glares, etc. Not really a fun reaction for my MC. Momoko at least responds to your remarks and you can act on them at a certain point. As for Masami, its like their way of teasing her, since she’s so sheltered, they get lots of good reactions from her.
As for their time period, I think the people there don’t mind at all really. Because of that one time in Book 1 when you ran into that guard and had to say what Masami was to you. If you say she’s your lover, the guy gets envious of you and even tries to tempt her away from you. Showing as its probably normal.
Even if she is a future romance option, I’m sure the author will keep it tame and tasteful. I’m pretty sure you won’t have a scene like you do with Momoko. Or if you do, it will probably be in the last book, when probably lots of time has passed or they put in a time skip of a few years or so. So not really a cause for concern.
But I’m thinking Masami is actually far older then she looks. And that her current petite looks aren’t really her true form. If you recall in Book 1 she lost the use of her Panda. Something happened to her to cause that, maybe whatever happened also caused her to revert to a child like physical state as well. She regained the use of her Panda and you see her older adult form in the dream you have in Book 1 as well. That is most likely how she really looks and her current form is only temporary. And she will revert to her adult self sometime in the future books. Anyway this is just a theory and may not be the case, but it is a possibility.