I fully agree with Tlaero on the importance of understanding the characters, and what it takes to win the game before starting. No matter what method is used to plan out the game, this key understanding of the characters, their goals, and the player goals is integral and absolutely necessary.
It may be something as simple as answering some questions to get the girl out of her clothes and bone her, like the Meet 'n Fuck style of game where the difficulty comes from memorizing tidbits or knowing trivia.
Other times the difficulty comes from a ticking clock and is about earning money, building stats and such like some of the Lesson of Passion games.
The goal in BEW, for example, is for Brad to exit a chaotic week in a better position than when he enters. Sex in BEW is relatively easy. Hopefully it's somewhat clear in the public intro that Brad is lonely and seeking companionship, and not just sex. He's tired of being stuck in the friend box, and some of us know that we can get shoved in that box, even after sex. Of course, he's seeking sex too, he IS male, after all.

The difficulty comes from having a lot of opportunities and challenges being thrown at him at once.
In my life, there seem to be long periods of boredom followed by bursts of pure chaos where everything seems to happen at once. The car breaks down, a family member goes to the hospital, the furnace stops working, get a promotion at work, and about a dozen other things happen, both good and bad, all within a week or two. Usually I start wishing for that boredom again.
That's essentially what happens to Brad in the game. Everything hits him at once and turns his life upside down. The difficulty of the game is navigating this and coming out the other side better than when he went in. Feast too much on the available bounty and like any greedy treasure hunter he could end up losing everything. Don't do enough, and he will end up right where he started, lonely and alone, doing the daily grind. Win, and he may have a new daily grind that is happier than his current one.
Once I understood what motivated Brad, then I added love interests. I had to figure out what motivated them. Why would Brad have a chance with them? What were they looking for and how is that compatible with the main character? Why would Brad be interested in them over the long term? Most importantly, what about them was not compatible with Brad, and would need compromise on from both sides, and is that compromise realistic? That's how the scoring evolved. When a love interest needs to make a compromise, they will need a high enough relationship with the main character to actually want to do it. Has Brad done enough, won enough "points," elicited enough interest in the girl for her to decide that whatever Brad is proposing is ok or not. For some compromises, maybe the score is too high, and the love interest likes Brad too much to do something as well.
Once I have the winning conditions, and the characterization, all the losing conditions pretty much write themselves. At that point, the game pretty much writes itself.
Wolfschadowe