Responses
Hes not "used to be straight," Robin had always liked Hangyul ever since their childhood. Their feelings were mutual, it was due to the trauma that erased his memories.
And no, it's in character for Robin. Robin is unapologetically straightforward and doting. If you remember the mentions of Robin's past relationships, he had been described as the 'guy who gives it all'.
Robin can easily through Hangyul's desire, and he's never one to shy away from his cute advances so he accepts it wholeheartedly. I'm actually surprised they haven't done it at this point.
Why does it feel so random
And quick?