Dude, the body reacts the way it wants to.. You'll get hard and then you'll get soft and then you'll get hard again.. This happens during a massage just like it happens during your sleep or during the day.. It doesn't matter.. All you have to do is lie back and enjoy the message.. No one will be offended or upset unless you grab it and say, "suck this!"..
Don't react at all if your cock gets hard.. Don't look at it, don't apologize and don't look at the girl rubbing you down.. Just enjoy.. Now if she says something then obviously you can reply.. The best reply is, "This feels so good, you're so good at giving a massage, thank you".. Maybe she'll flirt with you or grab your cock but it's all up to her.. Just get hard and leave the rest up to her.. If you're cool and not creepy then maybe you'll get some action on your next visit.. But getting hard is a good thing..