Stolen Partner
Looking for stories where the couple begins to dip their toes into cuckolding - but then gets pulled into it way deep, and ends with the female partner leaving the main male character.
There are some incredible stories about this.
One is Tsuma ga, Tanin no Mesu ni naru made -Short Cut Kyonyuu Tsuma Sasaki Misaki Hen by Haruhisky / Royal Bitch. The wife willingly fucks the neighbor for her cuck husband. It ends with her leaving the husband to marry and live with the neighbor. She is visibly pregnant, and she does so because she knows the cuck fetish is so ingrained in her husband and so she wants to make him happy. There is a separate story told from the vantage from the other partner.
Another one--which doesn't get enough discussion, IMO--is Fiancee fell into sex by Nekomi Koi. Male protagonist lets his fiancee go on "dates" with his boss to spice up their sex life. It ends badly. She finds true sexual fulfillment in the boss and leaves him for the boss. The final pages involve him officiating their wedding and watching them make love after the wedding.
Equally great is My Wife, on the Other Side of the Peep Room, by Nekomi Koi. Premise is a little creative: wife goes to see a sham psychologist, who immediately plans to steal the wife from the husband. He does so by 'unlocking' her sexual potential and manipulating her into slowly fucking him. Ends with the wife leaving the husband for the 'psychologist' and them setting up recurring meetings where the husband is forced to watch the two have sex as part of her 'treatment.'
If there are others, please suggest them.