Actually, yes you can. Micro evolution is just that, very small but observable changes that can occur within a couple of generations. There would certainly be a visible trend towards larger testicles if men who slept with lots of women had a clear biological advantage and were 'outbreeding' their competition the way that rats do. The fact this hasn't happened indicates men have been successful with their breeding strategy for thousands of years. Of course, they could produce all the sperm they liked and there would still only be one egg to fertilise per women, so they'd need to be pretty huge sluts for it to be an advantage.