From thee Tax & Business Law Commentary Blog comes this post about Dominatrix seeking a contract. Mr. Levine thinks:

Of course, the question that first occurred to me is: Is the "Dominatrix" an independent contractor or an employee? This has obvious income tax and FICA/SECA implications.

My first thought was this: might the contract be void as being for an illegal purpose? The commentator to Mr. Levin's blog answers the independent contractor/employee question correctly (well, correctly for Indiana). I had to some looking for my answer. The short answer is that it is not void for being a contract for an illegal act.

The longer answer requires looking at the prostitution statute (IC 35-45-4-2):

Prostitution
35-45-4-2 Sec. 2. A person who knowingly or intentionally:
(1) performs, or offers or agrees to perform, sexual intercourse or deviate sexual conduct; or
(2) fondles, or offers or agrees to fondle, the genitals of another person;
for money or other property commits prostitution, a Class A misdemeanor. However, the offense is a Class D felony if the person has two (2) prior convictions under this section.
As added by Acts 1976, P.L.148, SEC.5. Amended by Acts 1977, P.L.340, SEC.77; Acts 1979, P.L.301, SEC.1; P.L.310-1983, SEC.3.

Then just to be on the safe side checking the definition of "Deviate sexual conduct" at IC 35-41-1-9:

"Deviate sexual conduct" defined
35-41-1-9 Sec. 9. "Deviate sexual conduct" means an act involving:
(1) a sex organ of one person and the mouth or anus of another person; or
(2) the penetration of the sex organ or anus of a person by an object.
As added by P.L.311-1983, SEC.10. Amended by P.L.183-1984, SEC.1.

And, that is legal reasoning in 60 seconds. It does beg the question of why we lawyers think of such things when faced with has other more generally interesting features.