My [30f] new boyfriend [35m] is still married for visa reasons and I'm not sure how to proceed

I'm going to try to keep this as succinct as possible because I tend to babble.
Just to give you a bit of background, I'm a PhD in electrical engineering and computer science, and head up a division of R&D for a big tech company. Unlike some of my underlings (said with love and #NotAllEngineers, I know), I don't have any sort of social anxiety, I actually like and enjoy people and I'm good with them so I'm often in charge of explaining our research and tech to external parties.
Through this venue, I met this guy, M, about a year ago. He's a filmmaker who does a lot of our product videos, and I was tasked with making sure he understood what he was filming and took advantage of everything the tech can do.
We spent a lot of time together over the span of about a year and really hit it off. He's a great person and I immediately came to like him very much, and it seemed to be mutual. We flirted a bit sometimes but nothing crazy, until I started noticing that he'd make up excuses to fly up to see me and ask questions he already knew the answer to, etc. Eventually we started texting and it culminated in my inviting him to my birthday party where a drunken kiss was shared.
After that I asked him out and we had a wonderful date. He was a bit subdued the night of but still the kind, funny, thoughtful person I'd grown to be a bit in love with. We kissed again and I floated off on my little cloud.
The next day he texted me to say he'd had a fantastic time but he believed he had to be honest with me, could I meet him for a drink. Cue the anxiety.
Anyway, we met for the drink and he explained that he was married, had been married for 4 years in fact. I was flabbergasted but that wasn't all. She's from a country in South America where they met while he was filming a documentary, started seeing each other sort of casually and then she confided in him that she really wanted to come to America to work there (she's a chef) and he suggested they marry. They did, she got a green card and they tried to make it work as a couple but found they just weren't compatible so they split up about 18 months in. As she's not yet a citizen and with the current political climate, etc. they've remained married.
Obviously I couldn't believe that, which he understood, said he wouldn't believe it either but he liked me and wanted to prove it to me so if I could come down to LA, he'd introduce me to her and we could talk it out.
I remained on the fence but figured I had nothing to lose at this point so I made the trip, met his wife. She's actually lovely, a really nice person. She confirmed everything M had told me and expressed regret at making him do this for her but said he was generous and kind, and never complained, except for when he met me and knew he couldn't pursue it the way he wanted to. I also met her girlfriend, whom she's been dating for the past year so basically everything checks out. They live together but have separate bedrooms and really just peacefully cohabitate, keeping up appearances in case the ICE comes knocking.
He's admitted to me that it wasn't the smartest thing to do but they're good friends now and he doesn't want to be responsible for sending her back to her home country where she wasn't doing very well. There's a pre nup that protects them both, a written agreement filed with an attorney for when the divorce happens and a timeline all basically hinging on her getting citizenship.
That reassured me a lot and I told M I wanted to give it a try. He said he'd understand if I didn't want to be involved in what boils down to a possible felony but I figured I could handle it, probably naively not accounting for the amount of subterfuge it would involve. It's been a few months and I'm honestly really in love with him. He's everything I've ever wanted in a partner but, perhaps irrationally, I have a hard time with him being married, even if only on paper.
He cares about me, he's thoughtful, he communicates well. He's kinder than he has to be and he makes me a priority the way I make him one. When he's staying with me, we can be a little freer in the way we conduct ourselves in public but obviously that won't fly in LA because he might get recognised.
I know they have at max a year, maybe two if things go south, to go but I sometimes can't help feeling like an affair. That's probably unfair and we've talked about it, and he says it's absolutely not the case, which I can intellectually totally recognise but have trouble with emotionally. He suggested we think about getting a place together somewhere neither of us are recognisable, like NYC or Chicago, and just being a couple there but that's still just so sneaky.
I really like this guy but I'm having a hard time and I'm not sure I can sit it out that long. Am I being puritanical? Unreasonable? Do I cut my losses and move on before it's really serious? Help?
tl;dr: The man I've been seeing is in a sham marriage. He's been open and honest about it but I still feel a little dirty. What do?


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