I'm a masseur, and during training we learn that things like this are natural and will happen at times. How we react to it should be rational and depending upon each individual occurrence, different. For example, if I felt that the client had come for therapy with the intention of recieving sordid services, I would firmly but politely end the session and inform them that their behaviour had been inappropriate, and ask them never to return as a customer again. However, if the erection (or other natural bodily event) appeared to be completely accidental, most of the time I would completely ignore this and continue delivering the massage.
Other things that can happen include...farting and other gastric reactions, blushing, even queefing, as well as nervous reactions (involuntary jerks and moans).
Most of the time, I don't encounter anything untoward.
I suppose, in a roundabout way I am saying that if any of you got an erection during a massage with me, it wouldn't bother me. But if you tried to make me tend to it, then it would absolutely bother me.
Some therapists are very uptight and make people feel terrible over things like this, as I've learned from co-workers in the field. Personally I think it's bad practice. Accidents do happen, and as therapists we are supposed to behave professionally and do our best never to make a client feel bad for anything that might happen during service, even when something unfortunate happens.
Anyway, thought I would give my two cents since I work in the field.