Here is the full list of all 17 Horny Sekai achievements.
Complete first date with Katsumi
Complete second date with Katsumi
Become Katsumi's boyfriend
Complete first date with Masami
Complete second date with Masami
Become Masami's boyfriend
Complete first date with Elmyra
Complete second date with Elmyra
Become Elmyra's boyfriend
Complete first date with Catelyn
Complete second date with Catelyn
Become Catelyn's boyfriend
Become boyfriend of every girl in one save
Win a fight by using only one symbol
Defeat every enemy type in one save
Research every skill in skill tree
Use your first combo