Perhaps age gap is a trope that can become a fetish for some people or a total turn off for others, but it's a merely a fact of life. While I in no way condone the child brides of some cultures, where an eight year old is forced to marry a middle aged man, there is no necessity for a married or cohabitating couple to be within a certain number of days or months in age to be other than peculiar. I am eight years older than my wife, and my sister is eight years older than her husband. My uncle married a woman sixteen years older than he was, and they were a totally devoted couple until her passing after their 50th wedding anniversary. Was my family obsessed with marital age gaps? Hell no, we just happened to get married when we met the person we wanted to spend the rest of our lives with (36+ years for me). In this story, it is germane to the plot that Kanoko is older than her very mature "husband." Maturity (mental and emotional maturity, not body maturity) definitely comes to different people at different ages. My wife was far more mature at eighteen years old than I was for a number of years after that, and I have known fellow males in their thirties and forties who are still living out their Animal-House-style fraternity days.
My point: Taking advantage of children or a subordinate in a difference in power situation is sick and unforgivable. At the same time, romantic/marital love doesn't really count the number of days between a couple's birthdays. If a 40 year old man (or woman) deliberately sets out looking for an eighteen year old partner (of whatever gender), that's a little creepy, but if two mature adults of different ages find themselves in love, it's a totally different thing.