If ultrasound destroys sperm - why is it safe for a fetus?

Bill Gates has funded a new technology that uses ultrasound to cause infertility in men, a type of "ultrasonic birth control" that destroys testes production for up to six months.

Today I dare to ask the question that makes many new parents uncomfortable: If ultrasound destroys normal testes function, how can it be safe for a fetus? (P.S. Many ultrasound clinics refuse to provide ultrasound unless it is medically justified. I agree with that policy.)

http://www.naturalnews.com/028853_ultrasound_fetus.html