Actually, very realistic. No sane man on the island would go up to a girl like her and ask for sex (especially when the island's filled with busty women). He'd be arrested on the spot for attempted statutory rape of a (supposed) minor by the SHO or SS (FS as called in the manga). To think of touching one or talking about them would earn you a social execution by society. Even if she did say she's not underage, the people around her wouldn't believe her cause they would think she is just a little girl trying to act like a grownup, and even if she brought out ID as proof, the people of the island would think that if you got aroused from her body type, you'd look at other small children like that as well, thus leading to a social execution. These circumstances make it very difficult for Hinami to have any sort of relationship of the opposite sex.