But for those who are a bit more tolerant towards things outside their comfort zone, or who already possess a taste beyond typical vanilla/harem fare, there is quite a lot to enjoy here, even if it hardly exists as more than a "one and done" sort of deal.
There's an important distinction to be made between netorare and netorase (don't mix up those last two letters!), as Aisai Niki very much belongs to the latter group. Netorare, the source of many a pervert's anguish, can be defined as the heroine forcefully being stolen from the protagonist- often by some sort of douchebag that the audience is meant to despise, and so a part of the excitement is meant to be born from anger and jealousy. Netorase, however, involves the protagonist willingly sharing their partner with another man (or men), and potentially engaging in group acts with the girl. They're quite similar, but the whole "willing" part makes them entirely different experiences. There is no jealously or negative feelings meant to be felt from watching Aisai Nikki.
Rather, netorase is meant to be exciting because it breaks a sort of taboo. Society teaches us that marriage, and romantic relationships in general, are a contract where two partners are only allowed to share physical and sexual intimacy with one another. Breaking that contract is "cheating"-- betrayal-- and grounds for emotional trauma, the end of a relationship, and a whole, whole lot of screaming.
While cheating is absolutely disgusting and morally reprehensible, there is a certain excitement that can be felt when things are fully consensual. It would require a massive amount of trust, and, honestly, is not something that is ever to really work all that well in reality, which makes fictional pornography the ideal place to experience that sort of excitement. Seeing a cute girl break a social taboo and experience an ultimate sort of pleasure, in my humble opinion, is a hell of a lot more exciting than Mr. Boring Protagonist Dude boning everything in sight. It's an acquired taste, certainly, but many people do tend to dismiss it as the hobby of crazies and the socially incompetent when there are actually very valid reasons for a normal, functioning human being to possess those sorts of interests. The biggest misunderstanding people seem to have about the whole thing is that it's some form of self-hate or passiveness, when in reality, it can be (and often is) a form of dominance. It is also a sign of confidence, as it shows you trust your lover enough to stay faithful to you in the end. Sharing her as a form of light bullying or because you simply want to see her drown in pleasure can be a lot of fun if you aren't the type who is easily governed by jealousy. "Purity" is a bit overrated, I think.
Aisai Nikki follows a pretty typical storyline meant to cater to those with the above-mentioned fantasies: dude wants to see his wife being pleased by another man, and so he invites in a younger co-worker to do the deed and, later, help give his wife some nice 3P action. There are certainly some things about it that irked me slightly as they verged a little bit more towards netorare territory (the wife wanting sex with the co-worker more than her husband, or a sort of 'double' pregnancy, which is just...), but as there are only 22 minutes of screentime with absolutely no form of characterisation, it's hard to really care all that much. Those scenes are worth a laugh more than anything else.
The animation, like most hentai, leaves something to be desired, as there are more than a few sex scenes composed of little more than a few frames of animation being repeated. In some (particularly the ending DP scene), it's impossible to even see what's going on due to all the damn censorship. It's like staring at an image of a distant planet or some ungodly creature. Eroge and ero-doujins are a whole lot more subtle in that regard, it would seem...
Most importantly - and this is the thing that most netorase and 'swapping' pornography fails on - is that the girl actually enjoys what's going on. She's a bit reluctant at first - why wouldn't she be? - but quickly becomes more willing and assertive. Hearing the girl scream about how much she hates it and wants it to end is a huge mood-breaker for me, so it's very much appreciated that all that stuff is kept to a minimum. On the other hand, it also stays away from the old "mindbreak" trope (the girl going insane and becoming some sort of slave or sex doll), so it strikes an ideal balance between the two.
And, really, there's not a whole lot else that can be said about a short hentai like Aisai Nikki. Whether or not you enjoy it depends entirely on whether its story and character design align with your own idea of what qualifies as sexy. I couldn't make that decision for you. All I know is that I very much enjoyed the twenty minutes I spent watching it. If you're a big weirdo like me, hey, maybe you'll have some fun, too.
