Whether sex scenes include the main character being bottom or top is determined by the prescripted events. Note that there are third-party game guides available that explain the requirements for obtaining certain sex scenes:
- GALLERY - FANMADE_GUIDE by Redyoshi
- 【Tavern of Spear矛之酒館】CG解鎖、地下城副本、支線小任務攻略詳解(22/12/9再更新版本) - persis33的創作 - 巴哈姆特
In Chinese(Taiwan) - 【CG】矛之酒馆C/G画廊解锁指南（至0.23） - 哔哩哔哩
In Chinese(China)
Also, note that there’s no guarantee that a particular position sex scene will be available with a specific character in a specific plot route, future updates may change this, though.