I appreciate your concern that obtaining the information though a privacy violation might be the greater misdeed, as it often turns out that way.
But that's just not the case here. He was sitting right next to you! When you're next to someone (even a stranger) it's no big deal to glance at their screen or the name of the book they're reading, it's just -- it happens. You didn't snoop through anything to "catch him" doing something untoward, you couldn't help but notice what he was reading about.
When you bring this up, and you must, don't let him make you the villain, because you're not. And he's no stranger on a plane, he's your partner! If someone's sitting next to me and I want to look at something super private, I move! I don't fault them for noticing something that's -- right there!
No, you violated nothing and found this out by perfectly reasonable actions.
What he was informing himself on were strangers, men who's health status he did not know, and ways to have casual encounters with them. That's not just about him, that's about both of you (obviously.)
I don't follow that just because you haven't been intimate in a while, it makes sense if he's seeing other people.
There are open relationships, then there are anonymous ones in which neither knows the others health status, which are what one can expect from Craigslist.
The gay men AIDS numbers are ridiculously high in this country because many men fear that testing positive will render them responsible, should they have unprotected sex with another, for catching the very serious charge of straight-up assault with a deadly weapon. So they don't get tested. I'm not referring to you or your partner, but they're out there, and they don't give jack about what happens to anyone else, and early detection is the difference from living a normal life span with HIV with early treatment, or allowing it to become AIDS and take your future, your health, and your life.
I know you know all this, it's just beyond me the blatant disregard he has for informing you he's looking for (hopefully just looking so far) someone without cluing you in. You're right to be angry and you're right to not let it go.
It's also insulting to you as a compassionate partner, because it implies he couldn't come to you and tell you about this. And it sounds to me you're more than willing to discuss an open relationship, or make adjustments as need be, pretty much what a caring, loyal partner does.
You do deserve and need to know where you stand.
I don't see any other way than you've got to talk to him about it. It's too important not to; if not to him, it is to you. Let him know you're not against an open relationship. And let him know you're concerned for his health and your own. Tell him it's torn you apart knowing you have to confront him on this, you wish he'd have brought it up, but someone had to. And say it as kindly as possible; people open up to kindness, they shut down to accusations and ultimatums.
But I agree; you CAN'T unsee this! For his well being but especially your own! Nobody's worth rolling the dice every time you have sex with them.
And I'd completely put it out of my head (if you can) how you came by the information. If he says glancing at his laptop screen is an invasion of privacy, just say, "No, you were right beside me, I was hardly snooping. But I wish to God I'd never seen it." That's true, it's impossible to argue, and it'll show him the pain you're
feeling and the betrayal of finding out the way you did. Don't let him put this on "Well why were you snooping then?" You merely glanced at a screen next to you. I do it, everyone does it; don't even give that argument oxygen to live; it's not even an argument, it's an attempt to distract from the issue. And a lame one, at that.
I wish you well, and I understand how frustrating this got to be. But the way you found it out? I think it's a non-issue; I really do.
(My first little post against your over 5,000 of them!)
Well, my opinion probably won't hold much weight, but I had to put my $.02 in because I can see you're struggling with this. You deserve to know what you're
dealing with when it affects you as this could. Be kind, be firm, and be prepared for whatever answers he may give. Could he have wanted (subconsciously even) you to see it, in an effort to let you know his intentions. He may say he reads those ads for kicks, he'd never answer one or post one.
I've no idea, but be prepared for his answers. It's out there, so it has to be addressed.
You can't control his response(s). All you can control is how you handle them; without anger, without regretful words, and with all the kindness you can muster. (You'll regret it later if you don't handle it with decorum, calm, and sensibility.)
You can throw things against the wall later!
I wish you every happiness. :D