Your best answer is to tuck it in and when she starts to massage your thighs think about anything other than what she's doing.
See, this just sounds like it would completely negate the relaxation the massage is supposed to provide. frick it, they are getting paid to give you a massage and I guarantee you that they have seen hundreds if not thousands, depending on how long they have been doing it, of erections.
When I had my vasectomy, the nurse that prepped me was smoking hot. I wont lie, when she started putting that stuff all around down there to sterilize stuff, I got a boner you could have hammered a nail in with. She looked at it, smiled, and finished what she was doing and then walked out. Luckily the doc had just given me some of what I like to call "Don't give a frick juice" so not only did it not bother me, but I joked with her about it when they came in to do the procedure. She was totally cool about it, said that it had happened hundreds of times before and that it actually made it easier to prep so it didnt bother her a bit. Then the doc made some joke about her loving her job and then she made a smartass comment back to him. It was actually pretty funny.
I plan on going to get one pretty soon, my back muscles are so tense lately I need one really bad. I dont plan on giving a single frick about getting a boner. If I get one, so be it. If she handles it for me, even better. I won't get my hopes up though.