Sometimes it happens. Yet, many men avoid massage for fear this will happen to them, or get a massage but are unable to relax because of this fear. There is no reason to be embarrassed.
Sometimes men get an erection during a non-sexual, therapeutic, full body massage. Touch administered to any part of the body can activate the parasympathetic nervous system and moves the blood through the body, which can result in a partial or complete erection.
An educated, professional massage therapist understands this and it will not be an issue for him/her. They use appropriate draping techniques and genitals are covered at all times. If you are still concerned, wearing more fitted underwear (briefs or boxer briefs) will provide more support than traditional boxers. The therapist may offer to cover you with an additional towel or blanket for your comfort and privacy, and you may ask him/her to stop touching you for a moment if you feel that would make you more relaxed and comfortable.
Note: If the therapist feels that the session has turned sexual for the client, male or female, he/she may stop the session to clarify the client’s intent, and may decide to end the session immediately.