Simply because it's not the real Leliana doesn't make the overarcing situation okay Leliana or not. You can potentially recognize this is wrong but could go down a path thinking you are sleeping with the real her despite recognizing this situation isn't right and she's not acting herself. If someone is clearly drunk and not acting themself taking advantage of that situation for sexual desires is not right no matter what they say. The fact that it's actually a demon and not her doesn't take away that you as the player thought you were sleeping with her despite knowledge that something isn't quite right. The character is objectified for the players sexual desires while the player is also getting used by the demon. Not to mention potential terrible followup in the real world of "I didn't know I was in the fade with a demon but you were acting weird so I slept with what I thought was you go on with your day"
I think whether it's leliana does define if the situation is ok. If it was Leliana's actual body then it's physical rape, whereas the alternative is demon manipulation of the player(which is rather common). I don't have an issue of the possibility of the player being tricked being available alongside them spotting the deception as an option. The player can be deceived into becoming flemeth's slave and that's an interesting story element. For those Players who recognise it's a demon and willing sleeps with it, is that worse than being allowed to make all sorts of tawdry deals with demons in past.
Don't see how follow up in real world would would be terrible. May be embarrassing for inquisitor finding out they slept with a demon & to find out leliana has no interest in them, coupled with whatever penalty the devs hit the player with for being tricked.