I know it's been here for at least a little while. What I can remember. It's a short story, and I believe the sex is minimal or nonexistent — the focus is on the relationships.
I don't remember if they meet because they're neighbors, or if the husband and the single woman are coworkers (could be conflating different stories.) They find out that they all get along and start spending some time together. The single woman starts to realize she's falling in love with the couple.
At the end of the story, the couple have been looking for a new home and have found a larger house. They invite the other woman over to see it, then invite her to live with them as part of their marriage.
Thanks in advance for any help.