We've done the breeding/impregnation scenario a few times, as fantasy and role-play realization.
We didn't start the hot wife thing while my wife was still fertile, so it wasn't the "real deal" in the sense she had any risk of pregnancy.
One scenario we realized was recruiting partners off of Craig's List for the opportunity to impregnate my wife - we did this three times with slight variations on the theme, in essence pretending to have a sterile/low sperm count husband and a wife that wanted one more child before her biological clock ticked out.
We've done two "breeding" gang-bangs using similar set-ups - the gang part used the idea that my wife didn't want to know who got her pregnant.
These were all hot encounters, even without the real risk of getting her pregnant. Somehow the notion that a stranger is going to impregnate your wife makes it hotter for everyone involved - the other man is sort of competing, and the fucking is somehow more intense and intimate.