This was one of the earliest series that I've read, which pertains to Yuri in its explicit definition. Rather than a cutesy shoujo-ai or mild yuri, it goes in-depths in all respects. Dreams, goals, past experiences that led to current circumstances, human relationships. Now, I understand that people are entitled to their opinion but I seriously find Hell Clues' assessment that the couple are mentally unhealthy is...well.. bigoted. People aren't perfect. Manga seems to make other people a certain way usually but in this case, Maka Maka presents individuals with pasts full of good and bad experience. We only learn a bit about both Nene and Jun and how they came about to their present state. SURE it makes them seem sleezy/dishonest but isn't that realistic if you consider their past? Its a broken world with these two trying to make something out of it. They found solace among one another and in that sense, a true happiness. You, Hell Clues, only focused on the fact that yes, Nene and Jun sleep with multiple partners. Yes they may or may not be in an "official" relationship. But what about the partners? They were never portrayed as the "oh, I love you forever and ever." No. All, or most, of the portrayed men were in it for the sex. A little biased but not entirely wrong. I'll use the same imaginative you used.
Imagine you were in a relationship where your partner only uses your body. They don't particularly care about what you do, what you want to do, how are you, what you have done, simply put, they want to ravage your body. Yet you're identified as a couple. Society dictates that any other kind of relationship is abnormal. A sin. A mental illness. A passing phase. It may or may have been different at the start. He/she did show affection in courting you. But then it quickly degenerated into sex every time you meet. But you have a friend. A friend that cares for you and talks to you. Makes you feel alive rather than an accessory.
And so on. People, you, others, may exclaim their lack of sympathy for this couple by stating things such as "Coward" "Just break it off instead of cheating" "Needy" "Lying to themselves" etc But every case is unique and in this particular case, even if it is just a story, they are finding their own peace without really hurting anyone. There are also many forms of "cheating". Physical, emotional, and mentally are the broader terms. Physically is obvious while emotional and mental and more vague. Emotion may refer to trust (as in you rely on somebody else more so than your partner) or mentally you may think of somebody else more often. People interact. Its hard to monopolize someone completely without locking them away.
Actual review (I guess):
I liked it. The episodic way of narration generates a glimpse of their lives as they progress. Periods of time that seem to be more realistic than a never-breaking narrative. It started at a random point, unravels such that you learn a bit about their lives, then ends as would a rolling yarn ball...continuous. Its a story about two friends whom find solace within one another in a chaotic world. They are not perfect. Not by a long shot. But they are very human. They have problems and they deal with it (by committing to some action in a reaction, running away, or doing nothing...its still a response). They're both very sexually charged individuals (who isn't?).
TL, DR?
They're a believable realistic couple.