Ore wa Kanojo wo Shinjiteru!

When something valuable is in your possession, it's wise to never let go of it. Strong temptation and enviousness leads others to grab the love of your life away in an instant and at the same time, transforming the personality and identity of that individual.  

Betrayal arises from inadequate deep searching inside a person, physical appearance is only a mindless shell served as a one night stand. Failing to see that will result in a one way ticket to eternal pleasure and nothing else.  

These two provides same Netorare story which contain: 1. Dumb stupid girlfriend who easily give her self up. 2. Beta careless boyfriend which can't do nothing. 3. Charming third-person whose aggresive. 

In both anime The GF 'gets blackmailed' and go with another man. 

Hentai about cheating, there's lots of it and here's two examples, in Kanojo Wa Dare To Demo Sex Suru the girl is completely slutty from the beginning, while Ore Wa Kanojo Wo Shinjiteru is the process of her becoming a wife submitted to another man that isn't her husband (and the husband never gets to know it in the episode, maybe in the visual novel though), but there's lots of cheating and everything that that implies in both, go at it if it's your thing. 

The heroine involed in a "backstreet" behind her boyfriend's back and the boyfriend has no clue at all. Not recommended for vanilla lovers. 

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. 

Both are Netorare, where his girl friend is cheating on him......  

As a curiosity they almost came out at the same time, only by a month, but anyway, both are rather mild stories about cheating, in Ore Wa Kanojo Wo Shinjiteru the protagonist doesn't even get to watch her bride in the act of cheating and in Oni Chichi the guy practically sell her daughter, to later make like this wasn't the big deal, so yes, one gets to the netorare part the other not quite, but gets more to the part of the girl getting to like cheating sex, so both are practically mild stories about cheating. 

The protagonist watches how his love interest gets defiled by another guy, or at least in Mesu Nochi Torare, since Ore Wa Kanojo Wo Shinjiteru doesn't quite get there, it gets real close, but I guess otherwise people wouldn't buy the visual novel, there's still lots of cheating though, with the girlfriend or bride being reluctant at first to next being more and more into it, in both cases of course, there's also another girl in Mesu Nochi Torare, but the focus is mainly in the first one.