I've never come across this before. I agree with brone-j when he says it might be to do with the angle. If your wife has longer thighs than you have, it will mean that if you're both kneeling on the bed while you're doing it doggie your cock will be entering her at an upward angle - eg. towards her spine. This will mean you're jabbing her in the rectum. If she has little legs compared to you, your cock would enter her in a downward direction, eg towards her belly, where her uterus and bladder will be more in the firing line. Does it make any difference if she makes sure she's evacuated her bowels before you have sex?