Yeah I actually realized after playing the game again you can't resist completely. I thought you had to resist completely in order to beat the challenge, but apparently what you have to do is go through the challenge and you can win as long as your hp is big enough. for example, got hp to 400, went for the challenge and choose Kotone's boobs, lost 230 hp but won the challenge.
My problem is that my hp at the time was 200 so no matter what I always failed the challenge.