Yes, it WOULD be incest if Marty got together with his mom in 1955, which is why he made every effort NOT to encourage her and did everything he could to get him with his future dad, not only to avoid incest but to save his own life. The movie never presents either him or his future mom as unethical, only unaware. If he DID get together with her, not only would it be incest, but he'd end up killing himself by creating a paradox. Are you saying it's unethical for the movie to even bring it up at all? He did the right thing, that's what matters.
As for Clara, I don't recall that SHE was his ancestor or related in any way to him, nor did his sons eventually lead to him being born. Other than being from the future, Doc is just a man who fell in love with a woman.
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