You are if you are deceiving the other person in order to have sex with them. That is rape by deception. In this case, the Envy Demon was deceiving the Inquisitor by taking the form of Leliana before seducing them, thus would be raping the Inquisitor.
Deceiving isn't rape, it just being an @sshole. Many guys out at the bars on a weekend are there to pickup girls, not get all permanent relationshippy but they wont necessarily tell the girl that they are just there for the night. If she agrees to have sex with one of them because she thought it might lead to something longer-term, finds out he just had a good time and doesn't call, it doesn't magically convert into rape.
If you pretend to be rich and snag a woman on that note, seduce her and then she finds out you aren't rich, the deception makes you an @ss and her feel stupid but it doesn't make it rape.
You sleep with someone who either lies about their STD status or it doesn't come up, you find out days later the person has herpes, the deception doesn't make the sex rape. You MAY have a reason to sue depending on the lawyer, or maybe not. It makes that person a total sh*t but not a rapist. If they have HIV and didn't divulge, the deception doesn't make it rape, it does make it a crime but the crime isn't rape.
If you pick up a woman at a bar, get hot and heavy and it turns out she's actually a he, you may be pissed (and horribly mortified and embarrassed) but there was no rape.
I don't do dragony wizardy games so I cannot address the issue you speak to but if a drug is used, then that becomes rape. Deception in and of itself doesn't make a rape. It almost always makes an @sshole.