Originally Posted by ImaginaryIllusion
It is not a given...this is a worser case scenario...but it is possible. There is no reason in my mind why an established couple would be 'silly' to ensure some rules were in place to mitigate that risk.
Similarly I would not consider it silly for a prospective third to want rules to mitigate risks to them....since couples are twice as capable as behaving badly as a single.
Or you can chose partners that you trust will make choices that honor the relationship and behave well. Even in the midst of NRE.