Generally, you don't get sex scenes when you win in games that focus on defeat scenes. So unless the game unlocks the gallery on completion, there's not really a reward for making it through.
Edit: What I mean is, if beating the game and unlocking the Gallery doesn't unlock ALL scenes, then you're being forced to lose to the enemies to see the scenes. Which isn't only inconvenient, it's penalizing the player for being good at the game.