I think the money in this story serves as a catalyst. Miyako is shy and straight laced, and she might be repressing her sexual desires. The financial reward gives her an excuse to ignore the "rules" (e.g. "it's weird, doing it with a friend"). The extra money is helpful, but she isn't totally broke without it, which I think helps the story feel less exploitative. That's just my two cents.
Yeah she can get by without the extra money from these activities, it's just that she's now able to enjoy life more freely with more money. She can help her sister financially and buy the two of them luxury food, buy nicer clothing, so fourth. All while going on a sexual adventure so to speak haha