If selling is legal and sex is legal, why is selling sex illegal?

It is because the government can't control the money and tax it. 

Because it's believed to spread disease, and also result in the exploitation of sex workers.

Also, you'd run into issues such as.... 

Someone is unemployed and needs to apply for government benefits. As part of receiving the benefits they can need to apply for and take whatever job can be found for them that they can do. Oh, guess what. Sex work is legal. You can do that. We're not giving you any government benefits because there's legal work you're capable of doing.

You'd ultimately end up with a lot of people in strapped financial situations turning to sex work that they otherwise would not do. And just how much does the economy advance, innovate, etc., if everyone essentially spirals downward into becoming sex workers? It's better they at least stop with everyone working for Uber and Lyft.

Because it is felt to be immoral.  But it's not illegal everywhere.  In English law it isn't, though everything around it is to protect girls from being exploited. So be a prostitute who doesn't advertise and works entirely for yourself and that's OK, as long as you report your income and pay tax on it.  It's the best way really.  If a prostitute is abused or assaulted, they can feel safe reporting that to the police as THEY didn't do anything wrong.

Because the government doesn't get their cut

To subjugate women to men.

There are three main arguments for why selling sex is illegal.

1. because of the need to control the transmission of STDs. (though this could be more easily overcome by government regulation of the industry)

2. Because if selling sex is legal, then it makes selling "illegal" sex easier- such as child prostitution and human trafficking activities.

3. Because of the fact that it would reduce the crime of rape to the status of the crime of theft of services- Thus depriving the prostitute of the respect and dignity of human status.

It is legal where I live. 

The legislature says so.

Two reasons why selling sex is illegal are:

The government has no way to accurately collect taxes on sex for sale because it is a discrete and cash business without records because it happens behind closed doors.

Also, many young girls are used and made to be drug addicted for pimps to use them to make a helluva lot of cash, while paying the girls next to nothing.

It is a sleazy business and quite dangerous for wayward girls who are desperate. Sex trafficking is also a major concern and illegal. These girls are not in control of their lives and are passed around to men who use the hell out of them. Example : Jeffrey Epstien. Look him up.

Not all selling is legal. Not all sex is legal.