Anta Nankai Iranai is, first and foremost, a collection of oneshots all about "infidelity" in some form of another. It's about relationships and what happens when feelings stray.
Some are mild, where a person in a relationship develops feelings for another person (usually for the friend of their boyfriend or girlfriend). Others are outright cheating. The four oneshots basically approach the scenario a little bit differently, but they share a common theme of heartache and the difficulties of things falling apart when stuff like this happens.
In that sense, it sets the stories apart from the usual shoujo romance manga (which is a nice change of pace).
It's also the type of premise that I can imagine being something very angsty and dramatic if written out as a lengthier series (which could be a good or a bad thing, depending on your preferences).
The big downfall, however, is that it all feels bland. The stories aren't bad, but they're not particularly good either. They're, at best, mediocre. And outside of the premise, it feels unfortunately forgettable to me.
I think a large part of why is because of the characters. I don't care for any of them, so while the situations they come across are supposed to be sad, it failed at getting much reaction out of me.
For stories that are supposed to be about the heartache of feelings changing, it doesn't bode well when I don't feel the sadness of the characters. I know they're supposed to love each other, but I don't feel the love. I see the tears and I see the drama, but they all fell flat.
It could also be that the short length didn't allow for the stories to do the characters or the more complicated premise justice. Either way, the stories are missing the "oomph" that makes readers either cry, cheer, want to rip things apart, etc.
As I've said, though, it isn't a bad read. If you're interested in stories that explore themes of feelings straying, then perhaps this collection could do more for you than it did me. Just don't expect anything grand or feeling-shifting.
For me, it stands firmly in the very average rating of 5.