MC's first name is 恵, which by looking at it roughly 86% of the time people would think is female. I don't see the reading given, so it's very very likely to be the leading masculine-colored name reading for that kanji Kei (but could be another one like Satoshi, or an androgynous name like Yoshi.) (See a Japanese naming site http://www.feelbabyname.jp/name/name_list7.php?D1KJH1=&D1KJH2=&D1KJH3=&D1KAN1=%E6%81%B5&D1KAN2=&D1KAN3=&D1KHR1=2&D1KHR2=2&D1KHR3=2sex=1 for some evidence and links, which note that Kei is the most common reading for a boy, and 77% of the time people hearing it would think boy, but 86% of the time people reading it would think girl.) While Kei is a masculine given name, 恵 is only the seventh most likely single kanji used to write the name for boys, used about 3.4% of the time.
Anyway, he gets caught in a gas explosion in the restaurant he works part-time at, badly wounded, and the doctors seeing his name assume he must be a girl, and give him reconstructive surgery that also ends up changing his sex. This is of course fascinating at the all boys school he goes to.
Scenarios include him masturbating in the men's room, but the boys are peeking over the stall, and demand he help them masturbate too. They rape him. He tells his sensei he needs to use the women's restroom, but the sensei disagrees even after he says what happened, saying he is a very attractive women so why cant he understand the boys feelings. Finally the sensei has sex with him, asks him to not to tell anyone about this, and gives him a ton of money afterward (50,000 yen) for his silence. So he realizes maybe I could be a prostitute and make enough money really fast for a sex change back.
So then he has enjou sex with ojiisans, gradually saving money, but he also prostitutes himself to the school boys including the ones who raped him because they offer him money. Gradually he starts to enjoy all of this.
Then one of the men is the doctor who made him a woman in the first place. The doctor says he made his body to match his personal ideal, and that his dick is the best match for him, the one that causes the most pleasure. He says dont be a prostitute with other men anymore, from now on just do it with me and I will give you as much money as you need.
The doctor has sex until morning, traiining his pussy, until eventually all he can think about is the beating sound of flesh on flesh. He says he has a feeling the doctor knew he was a boy, but who cares, he loves having dick inserted, he desires it, he wants to be violated. That is the meaning of happiness.
The doctor gives him a ton of money and says lime I said at the end, it'll be a fun life won't it? He mumbles an affirmation.