This isn't rape. If you want to call it something, it's abuse of authority, but I wouldn't call it rape, as it seems pretty consensual.
If it was rape, the girl (or guy, if their roles were reversed) would have a noticeable aversion to the act, and we'd have though bubbles saying "No, no, no, stop, I don't want this, etc.". You "could" say she is being coerced into this, but she did accept becoming his slave, with all that entails, and I'm pretty sure the slaves in THIS universe can refuse a master.....I think (Could be confused with another isekai where it is possible).
But as Criver mentionned (Which I just read), there are a few grey zones that the author seems to have left to the interpretation of his readers. =/