Originally Posted by Robbie

I don't see anything wrong with it. Hell, it's nobodies business in my opinion.

Having said that...my guess as to why he used other people for sperm donors instead of himself would be this:
The guy was obviously taking massive amounts of female hormones. Not only to satisfy his desire to kinda be Dianna Ross, but also it kept his vocal range real high (similar to the "Castrato" of old times). I don't believe he could get an erection or have any fertile sperm taking that much female hormone.