That is where legalizing prostitution would lead to - among other things.
Okay we understand that but at the end of the day, the government is in all sorts of businesses that could be considered immoral (Porn industry tax revenues). Your argument that the government shouldn't collect taxes on immoral industries is already outdated by decades. Society is simply losing revenue that is going to the underworld when it doesn't need to whether we like that reality or not.
If, when defending your support for Donald Trump, and your response is,
"But but but... HILLARY!!!", then you lost the argument before you even began.