FAQ: Climactic Response and Control
An erection is likely to occur when you become fully relaxed. Premature or full ejaculation may or may not. The thought of this happening should not deter you from scheduling an appointment.
Again, such an occurrence will not be a surprise to me nor will I be offended by your reaction. Your appointment will not end unless you end it.
If you still have concerns, I have listed a few tips to reduce the chances of you reaching a peak during your treatment:
Urinate and/or masturbate before your treatment.
Deep Breathing Techniques (Google for details)
Kegel Exercises (Google for details)
If you feel 1, 2 or 3 will not be helpful, you may bring a protective barrier to safeguard your clothing.