Re: But... how can she have children if Vision is an android?
Well, a bit more seriously, consider that she has the ability to work magic at all because she was Chthon's #1 choice for a host body. Given that the Darkhold has turned people into werewolves just from reading it and the clay from Mt. Wndagore can be sculpted into puppets that mind control the people they're made to look like, someone tapping those forces to cause an extremely unlikely pregnancy doesn't seem that out of the blue.
Heck, Victoria Montesi found herself in a similar situation without even a nominal synthetic father being involved!