I've noticed more and more ads for providers that use the term "open minded." My guess is that it is a euphemism implying a willingness to engage in fetish-type activities. Is this correct?

For some it means they'll give you a blow job.

For some it means they are FS.

For some it means BBFS.

As a general rule, none of these generic phrases promise anything specific.

and perhaps also meaning non-judgemental, or "just bring it and don't worry about it, we won't laugh or tell if you don't"  
tee hee he