Context is a girl talking with MC ,looks like they're about to H
Girl: キミとひとつになるってことに……私、期待してる。
身体の奥、熱くなって……胸の鼓動が早くなって
なんだろう、この気持ちは……もしかして、一目惚れってやつなのかな。
私にはそんなこと、絶対にないと思ってたのに
キミの心が、綺麗なのがいけないんだよ?
I don't understand this sentence, my guess is "Why is your heart, it's beauty made me so hopeless/defenseless against you ?"
Hope someone will help me understand this sentence
While いけない can indeed mean “hopeless”, in this context it is used more in the meaning of “not good; wrong”.
Some examples from Kenkyusha J-E dictionary:
なんていけない子だろう.
What a naughty child you are!いけない, いけないと思いながら, つい彼の言いなりになってしまった.
Against my better judgment, I went along with him.君がいけないんだ.
It’s your fault. | You are to blame.
Especially the last example fits your context well.
Additionally, 綺麗 means not only “beautiful/pretty” but also “clean/pure (i.e. free of dirt/impurities)”, both in literal and figurative sense.
So the phrase could mean something like:
It’s all because your heart/soul is so pure
