uhh, I actually thought rape was being forced into sexual intercourse, and by that I mean directly forced. Since Mat never actually refused Tylin, it can't be called rape. He may not have liked every aspect of the relationship, and she was certainly taking advantage of him, but the moment he stood up to her and said he had to leave, she released him. This is another example of culture clash. I will agree that she takes advantage of him, but he also takes advantage of her, to a degree. He gets to live like a prince, for the most part, which he believes is fair trade, despite how much he complains. To call it rape minimizes the word, in my opinion. As for your arguments:
1. Not being married is not even close to a reason it could be called rape, since being married does not mean you must have sex with your partner, nor does not being married mean you can't consent to sex. That argument is ridiculus, frankly.
She did abuse her authority and used veiled threats, but only to keep him around. The sex part was never an issue for him.
As said above, we see that she lets him go the moment he directly asks her to release him. Since the A'dam completely enslaves the victim, I can't agree that what Tylin does is as morally reprehensible.