Being forced to work against your will at Mcdonalds sucks* being forced to have sex against your will is rape. Rape is considered by almost all countries to be one of the most heinous crimes imaginably.
I also don't have a source but would you agree that the amount of people being forced to work against their will at Mcdonalds is much lower than prostitution?
Your comment was trying to say "what's the line?" right? Like why should one profession be banned but another allowed if X percentage of people are forced to do Y against their will in each.
The line is rape, multiple rapes day in and day out.
*I assuming both still get paid some amount even though they are forced.