Where does “肛門性交” come from?

肛門性交 (Japanese) comes from Japanese 肛門, from Middle Chinese こうもん.

肛門性交 (Japanese): Synonym of アナルセックス

Definitions

Step	Language	Word	Meaning
1	Japanese	肛門	anus
2	Middle Chinese	こうもん	—

肛門性交 — Etymology, Origin & Word History | Etymology Explorer