I believe that some judge the concept of Romance that can be applied to Natsu under misconceptions.
We must first analyze his way of being, Natsu is impulsive, playful, teasing, someone who does not know how to take pressure situations that have nothing to do with fights.
We have seen this because we know that Natsu does not know what to do when he sees someone cry for reasons unrelated to physical injuries but to things caused by himself, likewise, Natsu is aware that a person is attractive, but does not know what is the love of a couple, we noticed that because Natsu is not romantic, with him we must analyze it based on the behavior of a growing child, a child that bothers those he loves.
I do not agree that Natsu treats Lucy as she would treat a simple good friend, Natsu has already had better friends before and did not behave like that, Natsu behaves like a child who is trying to attract the girl that attracts him. Attention, let who sees how children behave with the girls they like, will see that they tend to be quite cruel, because it is a way to get attention.
Natsu is not asexual, Natsu has what she does not know much about Sexuality since she has not been given a Guide about it and she does not have someone to help her understand that, it is very similar to Staz de Blood Lad, only that unlike Staz who had people who knew the subject and supported him to understand what he felt, most of Natsu's classmates are equal or worse than him.
The most mature of the Group is Gajeel and he is not given to help others on that subject.
The matter of which phrase is more canonical ... Gray's and Natsu's statements confirm to their ways of being what they feel.
Gray told Juvia that her body was his, it's Gray's own way of accepting what he feels for her considering that he's still a Tsundere.
Natsu told Lucy "We'll be together forever" which is a very Natsu way to show your emotions for Lucy, and coincidentally, it's what Lucy dreamed Natsu would tell her when she thought she was going to declare herself by the beginning of Manga , if none said "I love you" but ... Did they really think that with their way of being they were going to use that word if we consider that they have the subtlety of a mosquito?