I somehow doubt that Dembe had a relationship with the mother... I think since he was found by Red when he was 14, he might have started the education and everything pretty soon afterwards... Give him a year or two until he finally gets back to being social again and who knows, he might have met a girl and slept with her. What did he know at that age? Recall how he grew up...
Or what if he was some kind of prostitute before Red found him? What if Isabella's mother is a really bad woman? I have no idea but I think it very possible.
And Miranda, if we knew before how old Dembe was, I think we forgot again
Now we know and we mustn't forget again!