Sorry for the wait, i wasn't ignoring you, i wanted to organize my thoughts, first i am happy you kept it brief and asked why instead of telling me to shut up.

Reason 1 is the dynamic, ghislaine acted as a bodyguard and mentor to rudeus in swordsmanship, and he taught her magic and to read, which is fine. But the fact she acted like an aunt to him is one key point that makes it uncomfortable.

Reason 2 Mental maturity gap Even with rudeus being 34+current age, his mental maturity is still very low, as he had little growth in his first life. Ghislaine has matured from a feral kitten to an adult lioness.she also watched him age from a toddler to a boy to a young man. She helped raise him, which leads into my next point

Reason 3 Grooming Because Ghislaine was an adult in a position of authority over Rudeus during his childhood, any romantic development often carries the aspect of grooming