Cause it is the easiest way to create a cheating and/or ntr story. So in this case, they added the male mc so that they can turn from a story of "the daily life of a family of 4 sluts" to a story "where the mc cucks the husband who is barely mentioned and later on he himself get ntred when you reach an ending."
On a side note, I see people ,who enjoy ntr/cheating story, seeks for the cuck to be an arse, but all the ntr story I seen tends to show that the chad is a complete evil asshole, who almost all the time doesn't take responsibility of his child after impregnating the wife. I want to know if my perspective is true or in actuality that there is a large amount of ntr/cheating story where the cuck is the dick.
To be fair the MC isn't really ntr'd at any point in my book since he pretty much moves into a family to stay with them for a bit and then it turns out they are allt nymphos who pounces on him and anything with a dick. Then he has sex with all four of them in order and then you as the player are able to choose how the rest of the story goes. But right from the start the MC knows that none of the girls are really the faithful kind and if he wants to be with any of them then he has to know or expect them to sleep around.
The game has 5 routes / endings you can go from the start four of them involves always going for the same girl or scenes that involves the same girl and the last route involves picking different girls. Through out the game you will see them have sex with multiple other men so it's not like the MC doesn't know that they won't stay faithful to him. Then it will all end with a huge orgy involving the girls the MC + 4 random guys and a friend of the mom, the MC will be fucking the moms friend during the orgy, doesn't matter what choice you pick up to this point while the other girl fuck the random men.
After the orgy is over if you always picked the same girl then that girl will seek the MC out after the orgy and they will have a final sex scene. If the MC didn't pick the same all the time then the moms friend will turn up and complain that the MC wasn't focusing on her while they were having sex and the MC admits he likes all the girls and then he has sex with her one more time.
Then after this the MC has to go home but before then the girls all tell him he's always welcome to come back and fuck them whenever. After this the MC will be sent a letter and in it the girl the MC fucked after the orgy will have turned up pregnant so it's either one of the 4 girls or the moms friend then it just ends with the MC dropping the picture.
So at that point it's kinda hard to tell what will happen next I mean if it's any of the 3 daughters (except for maybe Sayoko) especially Kurumi chances are high that the MC would marry them or at least get together with one of them later on. Though with the mom or the mom's friend it's questionable what will happen since the two of them are already married and doing this all behind their husbands back. Though the MC will be moving back in with them in about a year I believe and if he only picked one girl she will approach him and give him some "encouragement" showing that she wants him to come back. But yeah the ending is just MC has knocked someone up and he get's so shocked about it he drops the picture.
Though to be fair seeing as most of the 4 girls fucks random guys pretty much every day or multiple times per times and always let them cum inside raw while not caring about contraceptives, chances are high it's not his kid. At the same time if he picks the same girl each time then it's hinted at that the MC fucks them raw pretty much everyday so there is also a high chance of the MC getting that one pregnant.
But anyhow I really don't think the MC get's NTR'd here since the MC from day 1 knows how the girls are like while at the same time he's only living there for some time before moving back. What happens afterwards when he's gotten a girl pregnant however is just left up in the air.
Though with having watched some of the hentai associated with this family. How I've understood the girls from both this game and the hentai I've read is the following, 2 of the 4 girls are, or at least mostly are single while 2 of them are in long term relationships that are always present in all the content.
Kurumi for instance is a huge nympho but she is seemingly pretty faithful or at least tries to be and wants to have a happy marriage / relationship but everytime she tries her mom / sisters seduces the guy she dates or tries to date and turns him into one of their sex friend. So while she's the one who tries to be "normal" the most and fight the nympho gene the most she's a huge nympho but at least she would try to be a faithful nympho, though chances are high if she hangs out with her mom especially that she will be lured into sleeping around.
Souko is a bit of an odd one in that she like her mom is a huge nympho but at the same time she's an absolute slob that wants someone to take care off and pamper her. She like Kurumi is single and has been shown to want a steady relationship but so far she always fuck things up by constantly cheating and the fact that she appears diligent, prim and proper at first glance being a seemingly succesful career woman but once the boyfriends see's her true self have all ran away. I believe there are some hentai chapters about how she wants a steady relationship but she's scared all of the one's that stayed away or got caught cheating before it had gone on long enough for it to become a steady relationship.
Sayoko is shown to like the mom being in a long term relationship, which is also shown in the game and while she loves her boyfriend she cheats on him all the time and seemingly get's a kick out of cheating on him while he's close by or while speaking with him on the phone. So while it is shown or well at least she keeps saying she loves her bf it also seems like she has a fetish to cuck her bf so chances are high if the MC ends up with her that he will probably just be a sex friend of hers though probably one of the more frequently used one's.
Mitsuko is just shown to be a super nympho who will pretty much have sex with anyone at any time as long as they pull out their D or even just when she feels a little horny, which is pretty much all the time. While she seemingly loves her husband, though at times it seems like she loves him mostly cause he's away a lot and bringing in a lot of cash to support her and enable her to just live for sex, she can't live with out a d inside of her for more then a couple of hours at most. She's pretty much just a black hole of lust and depravity and seemingly loves to turn ppl into the same kind as herself and increase her never ending list of sex friends. I really can't see the MC being anything else then just one of her many many many sex friends since you'd need a whole football team just to keep her satisfied, though he might be the one she uses the most since he lives with her and is always easy access for her...