Once or twice I've been to men who made it clear they'd 'help me out' if I wanted near the end of the session. I figure a guy knows how a dick acts, a woman, well, sometimes they can get the wrong idea. When I turn over, even if I'm under a towel, it's obvious if I'm hard.
It's more just a response to the good feelings all over than it is sexual stimulation. Much of the time when I get a massage, I get hard-for part or all of the session. Frankly, I'm just more comfortable being naked around a man and I don't feel the same pressure I do if woman is massaging me. I have had massages from men and women, and I prefer men because, in my experience, they put more pressure and just relax me better.