I think their relationship is supposed to be like what you would have if you were a kid, like, the holding hands type. In my eyes it's that Sana's needs for sexual exploration outgrow the MC's, so they kinda "mature" apart. Like, the MC still wants that "innocent" Sana, but Sana is deviating further away from that "version of herself" exponentially.
It's a great premise for NTR, MC's significant other growing to enjoy stuff that the MC cannot provide.
Anyway, I do understand your point, they barely exchange any meaningful conversation for them to actually be a couple, but I think that's just what their relationship is like, and the game wants to be humorous anyway so realism would kinda ruin that I guess.