What are victimless crimes? There are several ways to define this, with the best way, to simply state the obvious, a victimless crime has no victims and therefore the only reason that it is a crime is because the State has decided to make it a crime. The most common victimless crimes would be: prostitution, illegal drug use, and gambling. However, some people believe that euthanasia is a victimless crime, I don't agree and it is implicit that if someone assists you in committing suicide, that that is a crime, but even without assistance, euthanasia has a victim, which is the person himself committing suicide.
In regards to gambling, many people have an interest in playing cards or dice or sports betting in which they get pleasure out of the possibility of winning money and are willing to take the risk in doing so. Most states now have some form of legalized gambling, with regulatory authorities and licensing. Gambling that is on the up and up has no victims, because the choice to gamble is left in your hands. However, in the case in which a gambler is being cheated, for instance the cards are marked, or the dice are loaded, or the sport event has been fixed, there most definitely is a victim, and therefore that would not be a victimless crime, but in absence of such cheating, the choice to spend your money gambling should be your choice alone.
I am somewhat amused by illegality of certain drugs in America. The reason I say this, is that there is a misimpression in America that if a drug is legal that it is OK to safely take, and that if a drug is illegal that is unsafe to consume in any amount, but that isn't true at all, because some legal drugs are quite toxic and some illegal drugs are not. The State arbitrarily decides which drugs are legal and which are not and the decision should not be the States to make in the first place. The choice should be made by the individual and most informed individuals would have a preference to take drugs that are manufactured with controls, clinical trials, and oversights as compared to "street drugs". It is not for the Government to decide what you should or should not ingest into your body and certainly it is unfair and a waste of community resources to incarcerate people that use or take so-called illegal drugs. As for those that sell drugs in which they have not been licensed to do so, that should be a crime and there should be appropriate penalties for doing so depending upon the danger and dosage of the drugs sold.
As for prostitution, this is typically a transaction between two or more parties in which each performs their part as part of an agreed upon verbal contract. As long as each party does their part, it isn't the business of the State to interfere in the transaction. There isn't anything wrong with people or the State considering the act to be immoral, but I do take exception to it being treated as a crime though.
Crimes in which there is no victim should not be criminal acts. Adults should be able to make decisions as adults, we may not approve of all of them, we may not agree with all of them, but those decisions should be left up to the individual for better or for worse.