I've always been confused by this question. Barring the obvious explanation "When a mummy Tech Priest and a daddy Tech Priest love each other very much..." where do they come from?
...How exactly would somebody go about getting inducted into the Machine Cult? It seems strange to me that (on non-Forge Worlds) people are taught to venerate The God-Emperor, yet they would willingly turn their back on this to join some secretive organisation.. I mean would they just walk up to a Mechanicum Temple and say "Oh hai, can I has join yoo guyz? Kthxbi!"
...Thats another question, what would a Mechanicus Temple look like? Do they even have temples, or do they just huddle together in Reactors/Factorums/Tech-Support Offices/Internet Cafes?
I'm so confused, please can somebody help me out?