wait so there are no sex scenes whatsoever on a untainted natasha route? I took a strong anesthetic just to make sure corruption wouldn't increase and it didn't but she still switched to isha. see, the thing is, she should only switch to isha if she gets to max breast size. there is a dialogue seen when she's masturbating after her breasts hits full size where she devotes herself to isha. Based on context she should be sexable on the untainted route too, she'll just see it as penance or a way to fix the curse. this is again evidenced by her asking athia to forgive her during the sex scene and that she thinks it's the goddess testing her. also if you ask her about being a priestess after the blowjob, she says athia's gift is fading (her holy power is fading). During the sex scene, if you take her virginity, the mc asks her if she thought that this is exactly what the goddess wants from her, to sacrifice her most precious possession. heck she straight up says that she still trusts in athia during the scene. Again, more proof that she should still have faith in athia over isha. like I said, it's a bug and the developer will hopefully fix it for the next release.
if you're not able to have sex without fully corrupting her (if she isn't fully corrupted) then what's the point of going even cleansing the fountain?