Female primates, and only females, are known for especially loud noises during sex. Scientists have long speculated that these "female copulatory calls" aren't just a pointless quirk--they might have an evolutionary function.
Female primates aren't just overcome with passion, according to this hypothesis--they're telling all the other males nearby what they're up to. Attracting more mates during copulation is a way of inciting male-male competition, so that she's more likely to find a high-quality mate.
Moreover, by mating with multiple males, some researchers have argued, paternity becomes confused. That may not sound obviously desirable, but it decreases infanticide by males: Male primates are less likely kill young they did not father if they can't tell which babies are theirs.
One other idea for why female primates (potentially including humans) vocalize during sex: to get their mates to climax faster. In either case, it seems to be a way for the female to assert reproductive control.