"If I join a guild and the use parsing, am I obliged to report it?"
Yes, but they will eventually figure out its you, and you will be trapped in a cycle like that forever.
"Can I do raids with others that parse?"
No, that makes you a cheater by association and you should feel bad about it.
"Am I allowed to look at guides, rotations and discussions based on parsing on the official forum and elsewhere?"
No, because it was acquired through illegal means, meaning using them is against ToS and you should feel bad.