when i first started getting massaged after a week's worth of working out, i would usually get hard during massage. i was always completely naked and not covered by a sheet, so my (male) masseur could always easily see that i had a bone on. for a long time, he didn't mention it.
after a couple of months of weekly massages, i got a lot fewer boners. at that point, the masseur told me (without my saying anything) that it was very common with guys who weren't used to getting massaged to get some degree of erection, and that he had been told during masseur training that the best response was to ignore it until it got less frequent, and then, if they had a good relationship, reassure the client that it was nothing too unusual.
for a long time after, i would still occasionally get full boners during massage, and it never seemed to be a problem for either of us. the only uncomfortable part was that if i got boned up, i would drool a lot of precum, which was kind of messy. when he noticed the clear ooze, he never said anything, but would give me a hand-towel or a tissue at the end of the massage so i could wipe up the prejizz. i don't know how many masseurs are this cool with guys who bone up easily and drool a lot, but i would think he must be pretty special... and i've never used any other masseur, mostly because i really appreciate that we're both not freaked out by boners.
i have seen him for years, and now, after such a long time, it is very rare for me to get full erections during massage (unless i doze off, and then, it can still happen at times). i still drool a bit most of the time, though, and he still hands me a kleenex or a hand-towel to wipe up my precum with.