Its valid because male sperm swim faster, so if you give them all a longer distance, they will arrive at the destination first. And no, having sex that way doesn't suck...
I had a friend who had his 4th girl a month before my first was born, a boy. This guy was a real "man's man": great athlete, hunter, sports fanatic, dated about a hundred girls in high school and college (they nick-named him "the stabber"), and had been looking forward to passing the torch onto a son. He walks into my office nearly in tears and ax'd me "how did you do it?" I explained it to him. Working her G spot rather than ramming her cervix has its advantages...