Prostitution will never be legal in the U.S., I don't think. Legalization confers a sort of legitimacy to it that is against the morals of most Americans. It won't fly. If you decide not to participate in prostitution, then its safety is a moot point. The excuse that it'll happen anyway is childish. There are many awful things that happen in this country every day (rape, murder, illegal drugs, gang murders), but we don't have to condone them or make money off of them.
You are incorrect that it is illegal in the US... Prostitution along with many other criminal laws are held in the jurisdiction of the states and more specifically in each county or other local municipality, not the nation as a whole. Just because most states make it illegal doesn't mean that it is a national law. This is why Nevada allows (and regulates) prostitution in the form of legal brothels (the basis for HBO's Cathouse). There is no law in the US prohibiting prostitution, nor is there a law in Nevada doing the same. However, there is a law in Las Vegas, Navada against prostitution which is why all of the legal brothels are found outside of the city (and I might be wrong in that it is a county law, not just a city law).
I believe that this is the best way to handle these type of laws. In the US, we hold to the ideals of individual freedoms. Thus the country should be open to many things that may be morally reprehensible to some -- provided that it does not harm someone else -- and allow the states and local governments to dictate the laws that represent the ideals of the community. If someone feels that their faith makes prostitution a sin and therefore should not be performed, then that person can refuse to partake. If the members of a town also subscribe to this thought, then the town can create a law against prostitution. If in the neighboring town where it is legal, a person also wishes to raise their family in a wholesome environment, they have the choice by their own freedom to move to the city with stricter moral standards / laws.
Similarly if in Utah they decide that polygamy is in fact ok and perhaps moral for their people, then it should be legal there. And homosexual marriage should be legal in San Francisco. Etc. Also (to the point of the OP stating not to discuss sex between children), the age of consent (the age when the law feels that a "minor" can consent to having sex) is given up to the states. Most states actually have an age of consent set to 16 years of age, not at 18 as most people would think based on other age-of-majority laws. The national government, on the other hand, has set it to 18 for cases that fall into its purview. Thus although it is ok for two teens that are 16 to have consenting sexual intercourse in Washington DC, it is against federal statutes for them to use the mail or e-mail to share pictures of their experience with each other (the federal government's law requires the distribution of pornographic material to have the participants be at least 18 years old).
It is because of our federal government that we had to enact an amendment to the US Constitution to prohibit alcohol (and similarly to repeal prohibition), and why it would require an amendment to prohibit homosexual marriage, prostitution, polygamy, etc. This is also the case why some states are beginning to legalize marijuana. The only national laws against drugs pertain to the transport and distribution of drugs under the commerce clause (which gives the national government the ability to regulate commerce among the states) and since most drugs enter into the country from foreign entities and are moved between different states, the national government can create laws against drug trafficking.
When we say that the sex is between "consenting adults", I take this to mean that to both parties involved feel that they are not being harmed or forced into the decision, and so the freedom to participate is not violating the other person's freedoms. Once it does start infringing on one of the party's freedom, then it is no longer between consenting adults, and can be acted upon by the correct laws (eg rape, extortion, harassment, etc.) of that community.