How can Mormons claim they believe in the virgin birth if God had sex with Mary?
They change the definition of the word virgin. Mormons feel that they can still use the phrase "virgin birth" because God was an IMMORTAL being who had sex with Mary, not a mere mortal man. And this is exactly what Bruce McConkie, (top LSD theologian, and one of the Mormon 12 Apostles, died in 1985) said:
"For our present purposes, suffice it to say that our Lord was born of a virgin, which is fitting and proper, and also natural, since the Father of the Child was an immortal Being" (The Promised Messiah, pg. 466).
In other words, if Joseph had sex with Mary she would not have been a virgin, but since God had sex with Mary, she remains a virgin.
By "Virgin birth", Mormons mean that no mortal human had sex with Mary, but since God had sex with Mary, and He is immortal, she remains a virgin!