Why is prostitution illegal?

I am a prostitute. I have been for about a year now. I am 21 years old and a student in college. I work very hard in school and will be graduating. I do not have parents or anyone to help me financially. I was unable to find a job but needed some way to support myself. Long story short, I became a prostitute.

I have a small list of clients that I see on a regular basis, either at their home or mine. They pay me well for my time and I enjoy spending time with them. Condoms are ALWAYS used and non negotiable. I get tested all the time for STIs and HIV and am always clean. I am a productive member of society, I volunteer for several different organizations and projects. I like what I do. I am a very sexual person and have always had a healthy sex life.

If men want to pay me to have sex with them, what is wrong with that? Why is it illegal in most places in the US?