Before this gets too far off topic....probably a good idea to break down the OP.
My wife has been going for massages for many years.
This is a significant factor.
I always assumed it was a spa setting with multiple people in the room.
This is clunky. Perhaps he would substitute "room" for "spa" again and it would make sense. Regardless, we know he wasn't aware, wasn't made aware, or was deceived years ago, for whatever reason.
Turns out she is alone with a man for 50 minutes, door closed and shades drawn, covered by a sheet
I'm assuming he means towel or something like that, not a head to toe sheet with a shoulder exposed. Sheet is also clunky.
except for the body part being massaged, wearing only her bikini briefs, massage oil, everything except her bikini area and breasts are fair game for his hands directly on her skin.
See above.
He is a licensed "massage therapist" and I have been assured it is completely non-sexual, just for relaxation and, as needed, working out a stiff back or sore neck, that sort of thing.
These aren't his words... they are his wife's. That's what has been told, in haste, to reassure him once he caught the discrepancy between what he thought was going on and whatever is going on. People who lie even by omission will inevitably forget any small details of the lie. Small details like "I'm going to a spa" will matter to him...but since they aren't memories....she will forget them in weeks or months. It has been years at this point, he's not entirely certain what was said. When he pointed out the discrepancy....this is her reassurance.
A valid feeling.
I feel like I should have been told years ago exactly what the circumstances were.
The fact that he wasn't indicates either she knew he was uncomfortable with this....or didn't care.
Does this make anyone else uncomfortable?
Far less would probably make most men uncomfortable and they would admit it if they weren't shamed for it.
No. You don't have to be comfortable with your wife being greased up and rubbed down frequently for years. That probably wasn't in your vows. Most of the time "to have and to hold" is a part of wedding vows.
I don't believe it will lead to any actual impropriety or infidelity, but I don't like the idea of another man running his hands all over her feet, legs, thighs, shoulders etc. in private using massage oils. Thoughts?
You seem uncertain about some things. Apparently, you agreed to something else years ago...and you have every reason to disagree now that you understand what it is or what it is claimed to be.
Next time you would normally be relaxing with her or otherwise enjoying her company....ignore her completely. Do whatever you would if she weren't there. Make her say things twice before you respond...and immediately cut off any attempt at casual discussion. She should ask what's wrong in relatively short time. Your response to this should be....
"I don't know, something feels off/wrong, I don't understand why I thought you were getting a massage at a spa all these years."
One of two things will then happen because she either deceived you or not.
1. If she genuinely didn't deceive you, she's really going to want you to understand that this isn't an issue...and it's easy for you to ask for every detail, no matter how small, about the massages. Get his name, get address, get the location, his number, how much he charges, is music playing, are candles lit, do they talk during the massage and if so...what did she talk about. Every last detail. If she knows she's lying....
2. She's going to get super defensive. Remind her that she asked what was wrong...you merely told her. She may know that giving details will allow you to verify her story....and she may try to change the subject, play the victim, gaslight you into thinking your feelings aren't valid, or simply claim to not know answers because inside...she's panicking. Any of these responses are pretty strong indicators she's lying, after all....she asked what was wrong....you accused her of nothing. What is there to be defensive about. Most importantly, don't allow her to deviate the topic....any of these responses should be met with silence from you and a steady eye contact. Just sit...she's going to want to convince you so things can return to the way they were....but again, she won't be able to remember any new lies over time. If she plays the victim...remind her she asked you what was wrong, you explained that something felt wrong, and you were uncertain why you thought the massages were at a spa. You haven't accused her of anything, she has no reason to be defensive. You framed the problem as your understanding of these massages....and now, she's unwilling to give you details that would help you understand.
If she storms out of the room, out of the house....she's literally running so she can avoid lying more. She knows this is about to be humiliating for her. When she inevitably returns....don't speak at all. Do as you please, pay her no mind, no consideration. If she tries to wait you out....I'd suggest securing your finances and a lawyer and leaving. If she tries to resolve it in any way....any way at all....other than giving you the details you want to know....explain to her you know she's lying. You know it. It wouldn't matter if she claimed to be willing to give up the massages....because you wouldn't believe her. Explain it...don't argue.
Hope you get #1.