I'm paying off a mortgage, student loans, etc.
I'm saving up for a down payment, going back to school, retirement, etc.
You're nosy. What are you asking all these questions for?
I have an extravagant lifestyle.
I don't have any other marketable skills.
I have an expensive drug habit.
I was tricked or forced into this.
I like meeting punters and chatting with them.
I'm in it for the sex.
I've been doing this for years and by now it just seems normal.