And hell, of his sudden protectiveness towards her and her sudden horniness towards him were both driven by the bond, I could understand that. It's a bit of a cheap story mechanic, but it's porn logic, so whatever.
Trouble is, we never see that being established! The only thing we ever see that is directly implied to be caused by the bond is Zycris thinking Illya is beautiful and her blushing, when they hold hands during the bonding ritual. That's it. If the game wanted to imply that the bond spell created a much deeper emotional attachment between them, despite being strangers, then it needed to take some time to show that. Have the group explore the city a little bit, or settle into the castle and get to know each other, or spend time planning Illya's royal visits to the surrounding villages. And during these scenes, establish the two of them getting closer, thinking about each other, maybe finishing each other's sentences or some other little cues to demonstrate that the bond spell is bringing them together. That way, when he erupts in anger over her being injured(even if it's only in a combat tournament) it makes sense!
Or, if you want to minimize the new renders and dialogues you have to add, simply show the mark on his hand glowing when it happens. That would feel a lot cheaper and lazier, but at least establish that it was because of the bond spell.