Renpy works well for these type of games, simply as that. You aren't developing a AAA game for Sony, you are creating a porn game, nothing more nothing less. Second, I am not defending Summertime Saga, I was even opposed when the people decided to go for the infamous "tech update" and I said that it was impossible to do that in only 4 months with the lot of problems his team had in the moment and the lot of work it could take before he finished the game. This has nothing to do with the time, is more with the necessity of doing this change. There is a lot of people angry about it and with reason. Let that sink really deep in your brain.