While I get what you mean, she is an adult and she's made it clear to him that even if he would never reciprocate her feelings, she'd love him either way. This is, unfortunately, always gonna be an issue with isekai.
Imagine he wasn't an elf, but instead a teenage human, with the soul of an adult man. In the eyes of the entire isekai world, he'd be a child. If his strike zone is adult women, any woman he'd fall in love with and would in turn 'like' him, would be a creep. (even when we know he's an adult on the inside)
You can't win, unless the characters are the same species, share the same type of long life span or there's just no romance involved.
But, if it does turn you off of the manga, there is plenty of choices to read
(๑•ㅂ•)و✧
preach it, that's the thing about all isekai manga, ain't nobody gonna be happy with the romance cause either way they are way too invested in these fictional characters and find any kind of romance uncomfy.
romance will always be prevalent in like 90-ish something percent of all content and people need to just get over it. it's fiction the characters aren't real.

I would have probably enjoyed this story better if it either (A) had no romance or (B) the romance is appropriate... by appropriate I meant like probably falling in love with a girl who you didn't meet as a child because the way the romance unfolded here was creepy.
The immortal elf falling in love with a girl as they adventured the world together would have been cute and I probably would have enjoyed the story that way but the creepy factor is that he has a 30 year old human soul inside of that 107 year old elven body!! He has the maturity and age of a developed human man AND he met the girl when she was only a child... she was probably only 14-16 years old that time when he met her... he should have known better. We can't even give the excuse of "oh in elven years he is still just a child" that doesn't make sense because his soul is still that of a thirty year old man