It also often is taken literally, which is why it's "reverse" rape, rather than "woman-dominant" rape or some similar form. It's the same mentality behind false claims about domestic violence being male-dominated, when it's pretty equally shared between the sexes.
The OP I replied to seemed more targeted towards Sykes and his condition rather than our MC, which is what I was directly referring to. MC also enjoys it and is generally happy about things, once he got used to the mentality shift of their culture versus his own.
Harpies did use the MC's body while he was asleep, but he wouldn't have rejected them if they came to him openly as Sylphy gave her permission, and he doesn't treat it otherwise as having been raped, either, as he doesn't consider it to be carried out forcibly or under threat of injury against his will; in the LN it's more explicit that he knows what they're doing and he consents to the little game they're making of it, though he does start to stagger a bit and work hard to find something to boost stamina recovery so he doesn't end up like Sykes here, lol.
It was basically this way in the LN too; there are some more explicit moments later on, though, IIRC.