Some scenes are part of story, and so one-time only. But theres also some repeatable scenes.
After Gym you go to shower. You can either shower with your (female) instructor (not much for now, but Rome was not build in one day), which will raise arousal, or use empty shower cabin. If you use separate cabin, on high-enough arousal you have masturbate option. But right now I only once have chance to achieve this scene legitimately, and I miss the chance.
But theres other way to do that. Right now (as of 0.006, almost certainly it will be one of the first thing to change in future updates) stats such as «hunger», «energy» do nothing (also «alcohol»; it do nothing, and also never lowering after consuming, no matte what you do). You can actually never go to sleep (use locations with «wait» option, or simply walk between some locations to waste time) and «farm» arousal, until it is enough to trigger this scene (you can use shower, but must be excited enough to masturbate; otherwise, it is hard to catch, since arousal drops to zero when you go to sleep). Be wary, transport not working on night.
Besides this, you can masturbate in «your» room, in bathroom or in cafe’s toilet, just after one 2girl-shower, just do it before you go to sleep, or lust will reset back to zero. But it is hardly worth it, IMO.