Um, because cock size actually makes a difference in how sex feels? I mean don't get me wrong, I don't like giant monster cocks, there's definately such a thing as too big, but too small is no fun either. I guess if you're a guy who doesn't grasp the concept that women actually enjoy sex and don't just have it as a means to get men to marry them that concept may be hard to grasp, though.

Also I kind of sound like Goldilocks, but with cocks instead of porridge, don't I?