I'm not sure I follow the logic here. Is a man's best strategy to pass one of his sperm cells to each of 100 million women (every day)? And is a woman's best strategy therefore to have a different mate every month, at least until she gets pregnant?
It's all a matter of ecology. It might be best for a woman to stick with one male with decent-looking genes if the success of her offspring will be more likely with his protection and support. If she's working her way through several different males, they're less likely to spend time and resources caring for children that might not be their own. Of course, it could still be to the woman's advantage to mate with a particularly genetically fit male on the side, provided she can keep this secret from her mate.
For men, generally speaking, there's less cost in sleeping around. Sperm are replaced quickly, and it doesn't take a great deal of investment to spread them around. If they mate with somebody paricularly unfit and put no effort into caring for the offspring, nothing is lost, while you could always be lucky and the child is very reproductively successful. A woman has nine months of pregnancy for each child, though, and will have to care for it long after that if it's going to survive and be successful. As a result, she needs to be more discriminating about mates.
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