How, indeed ...
First, let us remember that these are made up people in made up situations. So there's nothing to get hung up about. I'm only too happy to try to clarify, for you.
Secondly, Neelix is probably around, oh ... 40, maybe? by the time we meet him? Around there. Now, maybe he's emotionally stunted, I'm completely available for that discussion, but he's been around a long time, either way. Quite aside from Kes' actual years, she's quite a sheltered girl. Yes, she wants to break free of that and Neelix did provide her with a way out, far and away from it all. Thank you, very much and all that, but they really should've parted ways, at that point. Or, at least just left it as friends ... and that would be just fine. Instead, it goes "Beyond" that ... way beyond! And that's where the 'use' comes in. He's too old for her, too different from her, which is why he's so possessive of her ... because he knows it.