Sex should just be illegal...
A transaction occurs when person A exchanges goods, services, and/or money with person B for other goods, services, and/or money.
If sex can be a service, and therefore be exchanged for money, goods, and/or services then isn't the act of consensual sex simply trading sex (a service) for sex (another service) and thus is prostitution? And that would also make rape equivilant to robbery, so there's that too...