You only need a handful of favor points five if I remember correctly. The trick is that there are some steps to get her to do that and the walkthrough describes it in detail. There is a bug if you started at day 40 or day 60 and you can never reach the sex stage.
Sex is not required to complete the story. So for the time being, sorry that it cannot be reached. If you used one of the intermediate starting points. There will be a bug fix in the next update.