I haven't come across a book, anime, or game titled 'SEX for FUN,' so I can't provide any details about its plot. It might be a lesser-known work or possibly a mistranslation or misinterpretation of another title. Sometimes, titles get altered in different regions, or fan translations can lead to variations that aren't official. If you're looking for recommendations in a similar vein—whether it's romance, comedy, or mature themes—I'd be happy to suggest some great titles that explore relationships and intimacy in engaging ways. For instance, 'Kimi no Iru Machi' is a manga that dives deep into romantic relationships with a mix of drama and realism, while 'Scum's Wish' tackles more complex and emotional aspects of love and desire. If you're into games, 'Dream Daddy' offers a lighthearted yet heartfelt take on dating and parenthood. Let me know if you'd like more suggestions!