Doesn't the fact that prostitution is illegal imply that pleasure is not a considered a legitimate and significant moral good? Prostitutes are said to be people who provide nothing of value to society. Nothing of value? Really? Perhaps this is because our society has a deontological system of values? In a utilitarian standpoint wouldn't it not only be moral to make prostitution legal wouldn't it in fact be extremely immoral to make it illegal since sex is extremely pleasurable and in a utilitarian calculus more pleasure equals more good?

I don't think the illegality of prostitution has direct implications for whether or not we think pleasure is a moral good. We might think that pleasure is a moral good, but might ban an activity that promotes short term pleasure because we think (rightly or wrongly) that it results in a long term overall reduction in pleasure. So, even a group of hedonist utilitarians might ban prostitution if they think (correctly or incorrectly) that it spreads STDs too much (including deadly STDs) thereby producing a net decrease in overall long term pleasure. Someone might also be in favor of banning prostitution because they think pleasure is of genuine worth, but merely of less worth than other goods (virtue, stable family relationships, etc.). You may also recall that Mill's version of utilitarianism weighs the 'quality' of pleasure and not just the 'quantity'. So, someone might think (correctly or incorrectly) that physical pleasure is of a lower quality than other pleasures and therefore should be weighed...