User blog comment:KatsushikaHokusai/NPC Expression Sheet/@comment-34981175-20180408155514/@comment-35303690-20180419041044

Ah, if I remember correctly there's an R-18 doujin that involved the Babylonia civilians to rape the Master's servant (which is Artoria Lancer) on behalf of Enkidu.

That's probably where joke came from.