(18 chapters) http://www.mangago.me/read-manga/saezuru_tori_wa_habatakanai/
(21 chapters) http://www.mangago.me/read-manga/rutta_to_kodama/
(17 chapters) http://www.mangago.me/read-manga/leopard_hakusho/
(18 chapters) http://www.mangago.me/read-manga/sorenari_ni_shinken_nandesu/
(27 chapters) http://www.mangago.me/read-manga/ten_count/
http://www.mangago.me/read-manga/heart_no_kakurega/
http://www.mangago.me/read-manga/doushiyoumo_nai_keredo/
http://www.mangago.me/read-manga/yume_musubi_koi_musubi/
http://www.mangago.me/read-manga/himegoto_asobi/
(spinoff)
http://www.mangago.me/read-manga/hidoku_shinaide/
http://www.mangago.me/read-manga/elektel_delusion/
http://www.mangago.me/read-manga/katekyo/
http://www.mangago.me/read-manga/honto_yajuu/
http://www.mangago.me/read-manga/konya_mo_nemurenai/
http://www.mangago.me/read-manga/koicha_no_osahou_tea_for_two/
http://www.mangago.me/read-manga/koisuru_boukun/
(check out her other mangas as well!)
http://www.mangago.me/read-manga/junjou/
http://www.mangago.me/read-manga/dear_green_hitomi_no_ounowa/
http://www.mangago.me/read-manga/kayashimashi_no_yuuga_na_seikatsu/
http://www.mangago.me/read-manga/hana_wa_saku_ka/
slow development so there's very little sex (it's an amazing story, though!)
http://www.mangago.me/read-manga/yuuutsu_na_asa/
another amazing work. there's more sex involved although still not much. both works are plot-heavy
http://www.mangago.me/read-manga/usagi_otoko_tora_otoko/
12 chapters so it doesn't quite meet the requirements but it's close! and it's super funny :)
Does anyone know a yoai that is long (longer then 15 chapters) ...that has sex