Looking for story about a lesbian couple getting pregnant by a friend but the wrong woman getting knocked up when they all have sex [MFF]

A lesbian couple asks a man (a friend?) to give them a baby.
They want to do it the old fashioned way and not just artificial insemination.
They have a decided on the woman of the couple that they want to become pregnant.
There is a bit of a ceremony when it takes place. They have the man change into white clothes and I think there were candles set about.
I believe that the other woman wasn’t going to watch or be there during the sex but they end up convincing her to stay?
That woman ends of joining in and has sex with the man also.
The man ends up coming inside of that woman and not the woman who they were planning to become pregnant. That woman says it is fine and they will try it again later?
Lo and behold, that woman ends up becoming pregnant?
I can’t remember if this was a tv show, movie, or a erotic story I read about. Has anyone else heard of this story?