Mako - does she look too young to you for a romance to seem appropriate? I have to say she never struck me as child-like. I'd never thought of it that way until I read that here just now.
I've never played a BH far enough to romance her (I keep meaning to, I really want to unlock Chiss), so I can't really say with much certainty, but I always thought she was meant to be more mousey than loli. Even if she is intended to be young I think it counts as "legal loli" since the story takes place over an indeterminate number of years and her lack of aging is just a limitation of the technology. That sort of thing bothers some people, but I tend to think of it as a "don't like, don't play" sort of things. /shrug