Ahhh that's the good ol' trope of invited by royalty my dood. By accepting the invite he essentially agreed to whatever it is she wants of him, her being a princess and him being a lower noble 🤷♂️
Reading between the lines is so bogus, and royalty can be such a pain in the ass abusing their authority to call someone over, yet have the audacity to act all humbled and think 'ah, they answered my invitation!' when in reality the person they invited would be publicly executed for turning it down.
And besides, she's an 'imperial princess'. If she's going to have someone be her 'prince' by political marriage with no romantic feelings attached, it's almost expected that he'd have a lover or two to have such intimate connections with,
especially if she, herself, is able to have lovers of her own.