Neelix & Kes had separate quarters.
I don't think they were sleeping together.
I kind of worked out that when they first came to the ship they were
living together and having sex. Then at some point Kes requested quarters of her own, followed by a gradual reducing is their love making and a general discontinuation of their romantic relationship.
At some point Kes pulled out the old "I hope we can remain friends" speech on Neelix. In terms of them being boyfriend girlfriend, Kes was the one who brought that to an end.
When we first saw Kes she was in a pretty bad situation, the Kazon were obviously physically abusing her, and it may have been worse than that. Neelix saved her life, whatever Neelix did over the course of the show that made some fans dislike him, saving Kes out weighs any of that.
Neelix was Kes's hero, and I believe initially she was deeply in love with him for that reason. But while she likely always felt gratitude towards him, the romantic love faded with time.