32M therapist here,

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Erections are 100% totally normal, its not our place to say anything or even bring attention to it, and the onus (in my opinion) really isn't on the client to explain themselves. We know what can happen when we are using manual manipulations to literally move blood around your body. We all should have 200+ hours of hands on work before getting our licenses. Your therapist should have been trained to realize that erections are completely normal and sometimes happen with massage.

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Now obviously if anything else comes with the erection, ie. physical contact from client to therapist, sexual innuendo conversation, even a weird glance. That is something else and I would be supporting the therapist 100%. If you're literally just lying there not saying anything, not doing anything but receiving your massage, you did absolutely nothing wrong and the therapist should be trained enough to deal with a random harmless erection.