Got the idea from telltale games though granted they to follow the story a lot more since they have to animate everything. In the games the player has to make a lot of decisions as they play some of which can have major impact on the story or at least seem to though they are limited but the story still follows a set path. The other great point about the games is that they are episodic so your decisions carry over in each episode. Stories on here generally have a huge degree of freedom but are usually lacking in the sex scene department in that you don't have much control over the sex. What if the opposite was true where the story was short but the sex scene was more interactive.
Could definitely work. I've played one of the Telltale games and I thoroughly enjoyed it, mostly because of the reason you listed with the decisions carrying over. One thing I'd be concerned about with breaking up the sex scenes is the flow of it. It can only be broken down into so many parts until it would be anti-climatic. On the subject of merged branches, there's an article (https://www.choiceofgames.com/2011/...te-a-long-interactive-novel-that-doesnt-suck/) that's an interesting enough read.
