Unsweet: Netorare Ochita Onna-tachi

Both are hentai that involve the protagonist's girlfriend cheating him with a douchebag from the same school to later become netorare, this all happens in a school setting, only that in Unsweet the girlfriend is a teacher and in Mesu Nochi Torare a student and there's another girl involved (since it's two episodes, instead of one, there's more time for story). 

As the names indicates, there's netorare in Unsweet: Netorare Ochita Onna-tachi, as well as in Kanojo Wa Dare To Demo Sex Suru, both happen in a school mostly and the producer is the same (T-Rex), so the animation, character design and art style is very similar is various aspects, Kanojo Wa has the boyfriend more willing to the whole thing, so maybe it's netori or something, but both are netorare at the end. 

If you know what netorare means then you should already know what to expect of a hentai with that word in the title, something truly Unsweet, and speaking of netorare, Otome Dori is an adaptation of a manga that has become popular for being one of the cruelest to the protagonist in these type of stories, so yeah both belong in the same sub-genre and are adapting manga. 

I'm not exactly sure why, but in the second OVA in the Oni Chichi series they decided to mix netorare, or maybe netori I guess to the story, they may wanted to go for a variety of sub-genres or something, anyway that's exactly what Unsweet is about, it also happens mostly in a school like Oni Chichi and the girl cheating is a teacher with a student, basically the roles are reversed to Oni Chichi, so there's that. 

They have a very parallel premise - A dark & sad scenario, in which a woman ends up drowning in pleasure, losing from sight what really matters to her: Love. The men are either naive and are bound to be heartbroken or scumbags who play the role of the breakers. *WARNING: Not for the Faint of Heart* 

They both involve the protagonist's girl getting snatched away from them, the beloved sensei for Unsweet and the wife in Ore Wa Kanojo Wo Shinjiteru, both of them also happens because of blackmailing and the girls look unwilling at first to later become completely submitted to the situation. Netorare or just cheating, they belong in the same category. 

A woman's loneliness is her downfall which shows in 'Netorare Zuma' where the strongest wins and desires can be addicting. 'Unsweet: Netorare Ochita Onna-tachi' tells you to not go too far with wrongdoings as it's almost impossible to turn over a new leaf when the cat is out of the bag. Scenes in both are short but strong which is what it's all about.