I think Masaichi's saying "we're childhood friends, we've always had great rapport and gotten along great, and we're compatible in a lot of ways and very close.
"Therefore, trying to apply some label to our relationship like 'boyfriend/girlfriend' and fitting what we have into a box based on what others see & assume, shouldn't be the way we go about things. Let's just be who we are together, because it's worked this well the whole time."
So part of it is something that crops up a lot, in manga and in real life everywhere--the notion of "labeling relationships" and trying to apply convention or custom to an arguably-singular dynamic between the parties in question.
We see this with her two friends--they both have their own ideas of what 'dating' means, and her short friend was trying to out their relationship as fake because her own idea was wrapped up in that psychic character's words.
But both of them were trying to apply what they thought 'dating' means to our FL and ML, and even though this relationship is "fake", the truth is it's just as real and valid and true as any. It might not currently have a romantic component, but there are plenty of dedicated couples who don't have romance, but have deep bonds and care about one another all the same. Or, not sexual component but romantic, and so on.
So here, he's basically saying "we put up a front for the sake of others, but maybe we don't have to. We've got what we have, and it works for both of us, so trying to perform a relationship for others' sake shouldn't be as important as whatever makes us happy"--or at least that's my interpretation.
Now the question becomes whether they cross from "deep platonic love childhood friends" into "childhood friends that like to hold hands and kiss and other acts of intimacy like sex". That's up to the two of them and whether they are looking for it with one another, and I assume that's the direction the story will eventually go.
But it sounds like Masaichi's of the opinion that what they have has always been authentic, and they should just focus on finding what makes them both happy, instead of trying to "play it correctly" for other people who aren't as important.
I could be wrong, but if that's the angle the author's going for, it's actually pretty nuanced and interesting--and a manga that takes the oft-used "childhood friends -> romantic lovers" concept and really delving into what it means to be one, the other, and how they are similar or different or change and grow, is neat to see.