I’m glad others have pointed out that you don’t just end up with a prostitute for a massage 
My XH was and is a fan of prostitutes.
He too had an “innocent” massage 
At Bubbles Spa, with all blacked out windows on a city side street in a dodgy area. Funnily enough, my second husband has massages too. Weird thing is, he booked in at PhysioDirect with a qualified sports massage therapist.
I had a similar situation to you, in that - there seemed to be nothing wrong with our marriage. So... how could I fix it?
I found out much later, he’d been using prostitutes since he was 19 (we married at 28) and never stopped - through my marriage to him, or his second marriage - second wife came to me when he claimed to her it “was the first time, honest” 
So the thing for me, was that there simply was no “rebuilding trust”.
Firstly, you ever hear the analogy of a broken plate? You can glue it, you can use it, you can even like it. The repair might be barely visible any more, and really strong. BUT... it will always have been broken.
Secondly, how do you ever rebuild trust? It’s this point about there having been nothing wrong in the first place. What can we fix? How can you know he’s not still doing it? I would never have trusted again.
My friend had an affair - cliché, with her PT. She went to counselling, was able to pinpoint with her husband the opportunistic element coupled with feeling crap about a post pregnancy body, which suddenly felt sexy again, which he hadn’t even commented on - ignoring her and their child for his hobby. They had a LOT of counselling. I really think they survived.
My XH offered me nothing - the same lies as you’ve had that it wasn’t often. The same nothing to fix situation. And an activity that can be easily hidden - how will you ever know he hasn’t done it again?
I left. Not the first time
more fool me.
My counsellor was shit too, but at least he wasn’t unprofessional enough to say completely unwarranted things about the strength of our relationship.
What fucked me off most though, wasn’t even the actual cheating. It was the fact he chose to play Russian roulette with my sexual health.
Don’t let anyone fool you with all the crap about how prostitutes are the “cleanest” affairs, because they get tested regularly. There’s a bloody reason they’re getting tested so often. And plenty offer “bareback” for the right price.
And simply the ethics of it. Any small amount of reading around the subject exposes just how often these are vulnerable women with little other choice - or literally forced with no choice. There are some women who post on here who are sex workers, who’ll tell you they’re happy and it’s a choice. And I believe them. Not so much the pimped out ex care drug addict 19yo working in “Bubbles” spa though 
That’s the man you’re with. I didn’t want that - even if I could trust he’d never do it again.