Well it certainly seems that he did not consider the implications of his actions if JY turned him down. It is hard to believe that he could be confident that JY would entertain his advances.
Did he think his life would carry on as normal if JY rebuffed him? That she would not tell her boyfriend about it, that his girlfriend would not find out. The list goes on.
I just cannot get my head around it either. Clearly his mind is not like the majority of the population, but did it just go haywire that night?
Self preservation kicked in after the fact, did he have no self preservation before the fact?
Totally baffled.