And this game tried to be sure the sex scenes served a purpose for their characters (except Halsin) and I believe they've done that successfully.
What purpose? I mean do you have deeper relationship in game through the romance? Does it actually do anything to drive the main storyline or would the story be essentially the same without it? Does it add a deeper interaction. Can you fight better with that companion at your side because your now closer and understand how the other thinks and moves? There is no real advantage to the relationships beyond maybe some small narrative points. If even having a friendship with the companions does nothing for the adventure experience then sex does less than nothing beyond what we are discussing.
The appeal for me is learning more about the characters. You get the extra scenes, and more dialogue options. You learn more about their needs, wants and vulnerabilities. That's a reward in and of itself. There does not need to be a mechanical reward or narrative purpose beyond that in my opinion.
