During the late 19th Century, women being forced into prostitution was a distinct problem. What “slippery slope” arguments derive from preventing people from being forced into something against their will? If anything, it works against gun control as it works against being able to do things to law abiding citizens, such as disarm them, in support of some greater good (women were often forced into prostitution to pay off legally-acquired debts, debt payment is usually considered a good thing).
If someone is held against their will, it is a crime all to itself. The inclusion of prostitution would be after the fact.