In San Francisco there are many massage parlors offering anything from handjobs to full sex. I have spent some time staking out a couple of places for research, and the majority of men are early 20s to mid 30s, with some in their 40s, and very few older than that. Most of them well groomed, attractive men. I was very surprised to find that, as I had always believed it was older men that frequent these places.
As for the OP, he seems to be conflicted about what he's looking for. Nothing sexual, and yet he's intrigued by the idea of an attractive masseuse seeing his penis. It sounds as if indecent exposure is what excites him.
My advice is to go to a certified massage if that's what you want, but if you are looking to flash your goods you would be better off finding a place that gives extras. There's no rule saying you have to accept anything other than a massage. Many of them are attractive, give a good massage, and are much more accepting of seeing a naked penis.