He isn't. There was a manga page(someone in another thread even posted it talking about how Obito's parent's died before his birth like Naruto which was essentially confirmed with the latest chapter) of Obito having a picture board, and on this board was a photo of team Minato something else and an older picture with worn edges of a man and woman holding a baby(aka Obito's only picture of his parents"Like Sasori's photo") The man in that photo did not look like Madara Uchiha.
Do you realize how old Madara was when he met Obito, if he busted a nut at that age he'd die.
Obito probably wasn't even older than the age of twelve, this isn't even a possibility unless you think Madara who has to live off of Gedo Mazo can get some pussy.
nuh but its interesting tho considering that madara was watching obito cuz in some chapter i remember zetsu said to obito after using mokuton (know i see why madara picked you) meaning that madara already knew obito and was watching him :yeah::yeah: