Here is my interpretation of the famale perspective: unlike men, that can potentially impregnate lots of women in a single day and keep doing so for many years, women can just get pregnant once in nine months.
This leads to a more selective choose of the partner, since they want their genes to be combined with the best they can find.
For this reason, seeing other women fucking their husbands may look like a confirmation of their good choice, they'd help the "best genes" spread (if the fetish includes cumming inside) etc.
This might work on a basic, "pimal" level, but it's more probable that more complex psycological factors actually come into play. >>91017670
Fuck no, I'll have a sad ending instead.