At least 60% of my clients are male. Many of them have shown movement or other sign of arousal. Half the time they are asleep. Some have apologized. Generally, we both ignore it. If someone starts moaning or grinding, then we have a real problem and you will be asked to leave.
Fear of erection is not a good reason to deny yourself the tremendous benefits of massage. It actually isn’t that common.
If you are worried, then tell the therapist. I had a client ask me not to work on his thighs because they are very sensitive and he was mortified by the idea of showing arousal. That took incredible trust and confidence. I simply thanked him for letting me know and it was never a problem. I’ve had other clients tell me they think of baseball, football, and people they don’t like if they begin to feel aroused.
I can’t even put into words how important it is to use good communication in the massage setting. It is your number one job as a client to “use your words”.