Because f**k logic.

Because Scarlet Witch is a witch and can do crazy stuff with her magical powers. That has always been the answer as far i know.
 
Also, mind the language. It's against the rules.

Vision is an Android. I dated an Android once and she was capable of let us say... giving birth... if you know what I mean. Oh...  
 
Anyway so yeah. He can cry remember, and crying is like the most important part of procreation. (or is that just me?)

It's part of a little thing I like to call PIC (Plot Induced Children).

" Vision is an Android. I dated an Android once and she was capable of let us say... giving birth... if you know what I mean. Oh...   Anyway so yeah. He can cry remember, and crying is like the most important part of procreation. (or is that just me?) "

Standard Marvel Rule if there is no logical explanation possible its magic and thats all you need to know

As far as canon is concerned it was the Scarlet Witch's deep inner wish to have children that created the two. She wanted children so the Hex-power made her some.

Wanda used the power of the Salem Six to impregnate herself with Vision's essence.

@wdchefdave:

Those two are the twins that disappered.

The 2 children she magically created were destroyed. However, their souls ended up in the bodies of Speed & Wiccan. As far as Vision knocking her up...don't player hate!  

@z3ro180: he is described as a synthiod he is not a so much a android as he is a cyborg almost plus the fact that witches powers can control reality. I hope this helps