The series glossed over what must have been a tremendous emotional toll on these men. The job of the fanfiction writer is to delve into that, and in the process we discover what becomes our reality for them. In mine, there's no room for parenthood. I wrote two stories that set up the scenario of Illya fathering a child with Marion Raven (A Fragile Heart (https://archiveofourown.org/works/250170) and One Time (https://archiveofourown.org/works/226343)
I think he does what seemed to be the best for the child, and for Marion. But, that's my world of MFU.