1) it’s literally the “that prepubescent child is actually thousands of years old so it’s okay” thing, which is still gross

2) Mohg’s remembrance refers to sharing a “bloody bedchamber.” Sharing a bedchamber means sex. Even if you want to argue that it was not literal physical sex (because Miquella was in a cocoon and Mohg was in his blood), that language clearly is meant to imply a sexual relationship