I will try my best to summary them but some of the details may not be too accurate.
Marina 1
1. Kenji (Husband) and Marina (wife) are married couple with a little daughter, Saki. Kenji had fantasy about lending his wife to other men for sex (netorase).
2. Kenji and Marina discuss it together and Marina does not reject the idea.
3. They decide to talk to a local supermarket clerk, Hiroto and find out he is a college student and recently broke up with his girl friend. They decided to go for the netorase experience.
4. Marina and Hiroto meets at the motel and have sex for the first time. Marina learns that Hiroto suffers from premature ejaculation and they run out of condom and lets him creampie her since she is on birth control pill.
5. Marina tells Kenji about experience with Hiroto and Kenji was so excited about it. Marina lies about letting Hiroto creampie her.
6. One of the meeting, Kenji decided to find them at the motel. He listens to Marina and Hiroto having sex but also find out that she lied to him. He asked whether they are having sex at the moment and she says no while Kenji is listening from the outside. She also lies that she and Hiroto has been kissing, but in reality she was refusing to kiss him.
7. On another occasion, Hiroto pushes Marina so much, Marina admits that sex with Hiroto is better than with Kenji. And also she admits that she "likes" him, kind of like when making confession to date each other. Also, Marina starts calling Hiroto by first name and Hiroto calls her by first name only when they make love.
8. Kenji and Marina has a hotspring trip planned but Kenji wanted to send Hiroto and Marina together.
9. Marina did not like the idea but end up going with Hiroto. Marina tells Kenji on the phone that she will call Hiroto her "husband."
10. At the hot spring, Marina finally kisses Hiroto and make love all night long.
11. Kenji decided to let them make love at their house.
11.1. If Kenji hides under the bed, he will see their legs (doggy style) and them falling asleep together like a married couple. And he also sees Marina has taken off her ring.
11.2. If Kenji hides in the closet, he will be found out and he will watch the full love making between them.
12. A year later, Marina is still continuing her relationship with Hiroto and it seems like Kenji is there in person often times to observe. He asks Marina whether he can videotape their sex and she says no.
Marina 2
1. Marina and Kenji decided to break up the relationship with Hiroto because he is failing at school.
2. Kenji meets a new neighbor, Yamashita who used to be an AV actor.
3. Kenji asks Yamashita about his netorare fetish and Yamashita agrees. But Kenji asks him to keep it a secret that he was an AV actor.
4. Marina and Yamashita makes love for the first time and he rocks her world, literally.
5. Marina lies to Kenji that the sex was not good and does not want to meet Yamashita any more. In reality, sex was so good, she feels it might jeopardize her marriage with Kenji (this might not be too accurate). Yamashita and Marina kisses when Yamashita tells Marina he will only make her cum if she lets him kiss her. She texts Kenji to get permission and she cums while kissing Yamashita.
6. Kenji convinces Marina give it more try and Marina have sex with Yamashita and it keeps better and better. Marina finds out Yamashita is pro at sex because he was an AV actor.
7. Marina and Yamashita makesa video together to show Kenji. In the video, Marina claims that her body not belongs to Yamishata only. At the end of the video, Marina has solo interview film by herself only and tells Kenji it was all an act and she only loves Kenji. She also lies in the video that she is getting creampied but in reality she does not.
I gotta go for now but I will continue later...
Let me know if you guys want me to put spoiler tag on this. I didn't put it in spoiler because Marina 1 and Marina 2 are older games.