I can't find this anywhere, probably because the names lead to biblical books and sites. If anyone knows the title or the author I would be very grateful! Here's what I remember:
It was a series of letters from one of Mary or Martha to the other. The letter writer is a housewife with two or three teenage children. The letter recipient is a lesbian and doesn't have kids. It possibly was called Dear Martha but I may have imagined that. It also may have started as a Guardian column, but I may have confused it with Dulcie Domum or Treasure. It was very funny. It was probably written in the eighties. I read it in the early or mid nineties so it may have been later.