Feel free to 'quote' (accurately please) absolutely anything I might have said to you in a private message.
I seem to remember things completely differently, but I'm not going to bother looking it up, because I have NEVER ANYWHERE said that my opinion was that Haley entered into sex with Klaus because she felt sorry for him. Nor have I ever felt or believed such a nonsensical idea. She KISSED him, because she felt sorry for him. That's it. Klaus shamed her into having sex with him, after the kiss, which wasn't even that much of a kiss.
Here is how Haley explains it in v0.90 (ep19.rpy):
Haley "He told me many times about him and his divorce."
MC "..."
Haley "One time..."
Haley "One time I hugged him, and gave him a kiss."
Haley "I don't know. I must've caught a corner of his mouth while I was kissing him on the cheek."
MC "..."
Haley "The next day, he was very upset."
Haley "He said that I was teasing him. That I was cruel."
Haley "That only sluts did that. Play with wounded men."
Haley "And that he'll tell dad. And..."
MC "I see."
MC "For how long?"
Haley "Three weeks."
Haley "Then mom put a stop to it."
She refuses to elaborate further.
Actually, that was pretty much how I always assumed it had happened: that she was conned into it; it had lasted for a while; Mom had somehow gotten wind of something going on between them; Dad knew/knows nothing.
Many 15-year-olds would have been mature enough to have told Klaus to go fuck himself. But Haley was not that person. She was too naive (and reliant on her brother) to fight back against Klaus.
I have also assumed that although her mind was repelled by sex with Klaus (he wasn't her brother), that her body enjoyed it... which is the root cause of her psychological problems in the present. EDIT: She has been on a gigantic guilt-trip.