Unfather and Hogu Hagyeongsu, though I detest both of those with a passion personally. If that's all you're wanting is straight up sex with not much of an actual existing story.
i feel like all of the mangas i've linked have a similar pattern lol. enjoy~ (≧∀≦)
demon x vampire
http://www.mangago.zone/read-manga/my_blood_and_soul_to_you/
literal pornography
http://www.mangago.zone/read-manga/walk_on_water/
rich guy x assassin
http://www.mangago.zone/read-manga/penthouse_xxx/
brothel owner x prostitute
http://www.mangago.zone/read-manga/down_and_dirty/
assassin x model (reverse couple/ALL TIME FAVORITE SMUT)
http://www.mangago.zone/read-manga/love_replica/
titan x human
http://www.mangago.zone/read-manga/the_titan_s_bride/
chef x sub-chef
http://www.mangago.zone/read-manga/pian_pian/?av=1
Glad to see I'm not the only one who thinks that way about it. Lol
yarin bitch club.....
http://www.mangago.zone/read-manga/sakurada_senpai_kaizou_keikaku/
Darkness hound.
Drugless sex.
Dragless sex 2.
Basically what the plot is based upon XDD
best yaoi porn
(please no shota :) )