Something tells me that it was never romantic on Kes' end. She agreed to it, out of gratitude for being taken away from the horrible Kazons and their atrocious manners. But gratitude only gets you so far. She discovered that if she didn't do anything about it. Neelix would be the last person in her life and that would scare any normal person...
I strongly felt that her feelings for him were romantic and I think it was shown displayed frequently, in small gestures and then consistently throughout certain episodes. Her devotion and ardor for him are evident in Parturition and Twisted, for example. Now, one can make the argument, and it was mentioned by the Doctor, that her awareness of what was entailed in a committed relationship was colored by the fact that this one was her first. She likely had never given serious thought before as to what characteristics she would look for in a significant other and how to define realistic expectations and goals as the bond developed.
Saying that doesn't trivialize or invalidate the affection that Kes expressed, at least IMO. Also, I don't see that she fundamentally has it in her to maintain the facade of a true relationship as a means of showing gratitude, even for being extricated from an existential threat. Loyalty was certainly something she greatly valued, but it wouldn't prompt her to put on a false front of feelings that weren't actually in motion.
The negative bias about the age difference and its implications is something I find rather superficial as well. While his personal history, motivations and convictions made Neelix a pretty complex character, I feel that his emotional persona was manifested in a simple, childlike form and on this basis there was in fact a symmetry, at least outwardly, in the pairing.
However, at base I think that he always had an intuitive sense that Kes was the one that embodied much more highly developed qualities of wisdom and maturity and was in many ways his superior. His Man of the Universe pose, quoting experience and savvy as justification for protecting Kes from her own supposed naivete, seems mere window dressing to mask his insecurity and lack of self-esteem as well as fear of losing the love of this deeper, and in his sight, more worthy soul.