Seems the 3 endings are tied to:
Ending 1 - Get fucked by any enemy that can impregnate Rose. Seems this also includes getting grabbed, but escaping before the enemy finishes.
Ending 2 - Get fucked by nothing except the werewolf. The only enemy that can't impregnate Rose.
Ending 3- Never get fucked by anything.
The code of this game is pretty scuffed. If you wanna get to the endings quickly, you'll need to first start from any stage, get a game over to be able to quit out anytime with the 'Enter' key. Next, start a new game, get the infinite arrows (optional), quit out, start stage 3. Play normally, avoiding all grabs until you reach the werewolf. From there, you can pick the endings based on:
Letting the extra dogs fuck rose (ending 1)
Letting the werewolf fuck rose a number of times (ending 2)
Just killing the werewolf without any grabs (ending 3)