I like it so far, even if I feel frustrated by the training being more teasing than sex. Maybe add some practice "training" sessions with Vi later so players can work out their frustrations at the beginning of the game.
Speaking of expanding training... any plans to add training sessions for the Demon Queen? Perhaps a gender changed Hero?
Heck, I can even think of a plausible scenario for the latter that doesn't involve already having already beaten him: Hero is frustrated and horny, Vi says to just get changed into a girl as the Goddess didn't outlaw having sex like that. The shocked Hero asks Mars if that's even possible and he says it is, if not as easy as she's saying. It'll take some research and may have side effects even if temporary (ie sexy curse mechanic like the girls). He goes for it and then 'she' gets trained by Mars until she doesn't want to change back...
PS: I also noticed the bug where clicking on the Heart has no way to exit the new screen. Rolling back with the mouse wheel works, but I don't think that's intended.