RPGM - When I see RPGM tag, I usually instantly skip a game. RPGM means ugly graphics, terrible UI, no mouse support, boring and repeating console battles, big empty maps and so on. In short, big boring time waster for few sex scenes. I preffer even RAGS (which also terrible) to RPGM.
Flash is thankfully dead. Although it can give OK short games with animations, I never liked flash, mostly because it lacked right mouse click feature.
HTML is actually good option for a lone programmer or small team as it possible to create quite good & complex game with it.
RENPY is also good platform for a porn game. Although it seems that RENPY is primary used for VN, it seems that it has quite a possibilities. I don't like VN format, I like to have sandbox feeling, meaning sense of freedom and options. Some RENPY games managed to do that.
Unity/Unreal are best options by far, but as porn games are usually fan or small projects, it's often better to turn to something easier such as RENPY or HTML.