If I With You All Night Could Be - David and Jonathan, #7

This is the story of Tom Appleton and his gay partner Luke Singleton-Scarborough. After several years of living in Italy, they are now established in their jobs and would like to start a family, but Italian law does not allow gay men to adopt children. When Olivia, a technician in Tom's laboratory is abandoned by the man who got her pregnant, Tom and Luke offer to become legal guardians and foster fathers of the baby.
They care for her during the rest of her pregnancy and are present at her delivery. When the baby is a year old, Olivia asks them to give her another child. After much heart-searching, they agree that one of them should attempt to father Olivia's second child. Olivia chooses Tom to start, and after several months of trying, Tom successfully fathers a baby boy. As the two children get older, Olivia decides that she would like Luke to give her a child.
This requires much more effort and actually necessitates them going away on a sort of honeymoon before conception is actually achieved. The book describes their interactions with Luke's biological family, and with his parents and grandparents in England and the rest of the book describes their family life. While much of the sex in the story is between the two men, there are also heterosexual scenes in their attempts at fatherhood.
The men conclude that they are really bi, and find that they have come to love Olivia as much as their own parents.