I respect your opinion because what you said is partially true from a realistic point of view , but why do you think Grey deceived everyone, because he got an opportunity to experience what he longed for in his previous life, a family.. Surely it would have been easier for him if he disclosed the situation earlier, but this is a fantasy reincarnation story we are talking about, so it was bound to have a plot like this.
As far as your opinion regarding the romantic relationship with a minor, Grey was against this too in the beginning, he resisted a lot because he was aware that his actions are wrong, but you just can't help falling in love, now again my opinion is based on the fact that this is a fantasy fiction we are talking about and not an realistic approach to the situation.