I agree with what's been said, and I'd like to add that there's a difference between a client who's in it more for sexual gratification and one who's in it for therapeutic benefit.
The former tends to give off a weird vibe, tug the sheets up or down to expose certain areas, move/squirm certain ways, make strange comments, or even strange suggestions, etc. The latter is genuinely interested in healing and, like vgambini, often very embarrassed. This is unfortunate, because he's unable to completely relax.
How I handle it is this: arousal is normal, and quite acceptable. I expect my clients to view me as a healthcare professional and not ACT on the urges that might've gotten stirred up. If they simply lay there and relax, then that's totally fine.
I discuss this with my clients during the consult for the first appt, and I offer them the suggestion that if they become aroused and subsequently embarrassed, then I have several towels I keep handy to throw over the area (which is already draped with a sheet). That way, my male clients are less self-conscious and it's out-of-sight, out-of-mind for the therapist.
Just my .02