I don't know about it in this game. But there are games where the sex happens fairly quickly but that doesn't make it a romance. Maybe just a fling? Or one night stand.
Like Morrigan in Dragon Age Origins you could sleep with pretty early on.
Morrigan was way harder. There's no comparison. Increasing the approval rating took a lot longer to get them to a point where the romance-unlocking dialogues popped up. Even if you spammed her with presents. And then you still had to choose the proper dialogue to have her develop a romantic interest.
Unless my memory is failing now.