This kind of situation is likely to have a very harmful impact on a marriage. Emotionally, it can lead to a great deal of confusion and hurt. The spouse who didn't initiate the idea may feel objectified or used. Socially, it can also lead to a lot of stigma if it becomes known. There could be family members who are disappointed or ashamed, and friends who distance themselves. Financially, if the marriage breaks down as a result, there are costs associated with separation or divorce such as dividing assets and potential child support. Overall, it's a situation that has the potential to disrupt every aspect of a married life.
Well, it can have multiple negative impacts. Firstly, there's the issue of jealousy. Even if both partners initially seem okay with it, feelings of jealousy can surface later. Secondly, it can change the power dynamic in the relationship. If one partner was more into the idea than the other, it can lead to feelings of being pressured or coerced. And thirdly, it can cause a rift in the emotional intimacy between the husband and wife. They may start to see each other in a different, less exclusive light, and this can gradually erode the special bond that was there in the marriage.
It can completely destroy the marriage. Trust, which is a fundamental part of a marriage, would be severely damaged. The husband may feel betrayed, and the wife may also find that her relationship with her friend has become irrevocably altered.