I thought it might have had to do with losing any fight, but I never checked after losing because I only ever lost the extra girls that you got for free, and while their overworld models were in horse bondage, two of them were already in horse bondage on the overworld sprites when you first got them, and the other was already clasified as a 'meat toilet.'
Other than Asagi (the one from another H-game that I recognized), they're really weak though so I think I made the game harder on my self unintentionally, lol. Asagi's really good at spamming normal attacks (she hits like 6 times, as I said, for about 130-190 damage apiece) but has no skills to speak of except really weak healing spells. I knew of the girls you could buy, of course, but figured they were going to be as weak as the two extra girls I got already and were there in case you wanted someone else. xD
Don't make that mistake like I did.