People don't like RPGM because it artificaly makes game longer. In RPGM you are wasting a lot of time on walking from point A to point B and often doing boring and repetable stuff. After that boring part of the game you have your reward (a scene) where you read text like in visual novel games. So to get scene in a game like this let's say it's 20 min of doing stuff, and than you get a 5 min scene.
In ren'py games you just have story in which you make your choices. If game in ren'py is good than you also have some freedom (sandbox element) and your choices matter. In ren'py games (if they are good) you don't have much grind and you only enjoy a story and scenes. So you play 10 min and in that time you can get 2 or even more scenes.
The main point is that your play time in RPGM games can be 4x longer than in ren'py games, just because of this walking from A to B and doing repetable stuff. Than you get rid of this stuff, game doesn't become any worse, just the opposite it becomes better because it doesn't waste your time.
Of course there are good RPGM sex games but they are highly concentrated on sex scenes for example battle fuck games or other games where you don't have to grind and fight monsters for 30 min to get a scene.