even though--as pointed out--there is a massage section, I thought I would offer my two cents worth here...
I have been a massage therapist for the past 6 years, and erections are an almost every session occurrance. Sometimes, it's caused because the client is turned on by me; but more often than not, it is the cause of blood being pushed toward the pelvic area, with the client also responding to touch in the usual "erogenous areas."
Most of my clients I assume are gay--since I live in SF--but in the past I have had certifiably straight men who get hard. If you ignore it as a therapist, no one gets embarrassed. A CMT without tact might comment, but generally a male therapist--whether they are gay or straight--will totally understand. Most of us can remember back to when even the wind blowing or the thought of changing during PE classes would cause a hard-on.
Try out some male therapists...and if it makes you more comfortable to mention the potential embarrassment upfront of an erection, then do it.
Trust us, a busy CMT has seen it all.