From what I've seen and the studies done, whites have larger dicks on average.
I think what happens is that women reach their sexual peak later on in life and their husbands just aren't interested anymore. I know I'm not. I only want sex about every two weeks or so while my wife wants it every damn day.
When I was younger, in my early 20's for example, I wanted it every day and my girlfriends only occasionally.
She likes blacks better for sex because she says it makes her feel more slutty and naturalistic.