I remember reading this doujin years ago where this middle age looking otaku lived by himself in an apartment. He was a big guy with a pot belly but really muscular arms. I think the plot started with a loli-ish girl falling through his ceiling and she finds all his otaku stuff in his room. From there, she wants the otaku to teach her weeb stuff because her family is strict or something. She visits like everyday to play video games until the otaku tries to grope her at one point. At first, she got mad but eventually they start having sex frequently. Around the end, the girl ends up telling the otaku that she's had sex with all the boys at her school and that she's experienced. That's about all I remember but I think there was a wholesome ending too.