Do you agree that sperm competition might be a vector for natural selection?

Sperm competition, or sperm warfare, posits that different males’ individual spermatozoa compete (kind of like sperm armies) inside of a female for the opportunity to fertilize an ovum. Many variations on the theory exist, but common themes discussed include the biological imperative of male and/or female promiscuity, functional differentiation between spermatozoa (e.g., being able to block another spermatozoon from fertilizing an egg), carefully primed ejaculates depending on the type sexual encounter or partner involved, and the possibility that sperm competition makes it possible for human beings to behave cooperatively while competing sexually. What do you think? If some variation of the concept of sperm competition is true, what ramifications does it have for sexual ethics?