On the surface, yeah, it's pretty bad, and if Neelix was a human I wouldn't excuse it (cough cough TOM), but Talaxians had much more cultural exchange and knowledge of the Ocampa. I can criticize the writers for not properly dealing with the squick factor, but in-universe I have to give characters the benefit of the doubt and take them at their word.

Ocampans really do mature and accumulate knowledge and understanding at over ten times the rate of humans. They have passed all galactic cultural standards for being able to consent, and are granted the ability to choose for themselves. They clearly also have some telepathic abilities, so it's likely those come into play in passing on knowledge and understanding from one generation to the next, allowing them to progress through maturity exponentially faster than most other species.

And since Talaxians have cultural history with them, Neelix has enough of a rich understanding of that dynamic to make an informed choice that has nothing predatory about it.

Kes could out-maneuver almost anyone on board both intellectually and emotionally, but she also has enough gravity to not seem either fey or condescending about it. I don't see any power imbalance there. If anything she's the one taking advantage of Neelix in order to escape the drudgery of her own life. She cares for him, but it's obvious from the eventual outcome that she doesn't really see him as a life partner so much as an opportunity.