Update:
Two days ago, I confronted my wife about my suspicion that she was having an affair with Jason. As expected, she vehemently denied it. Since then, she's been giving me the silent treatment, and even our kids have noticed, but we've kept it to ourselves. I've also noticed that she stopped spending time alone with Jason and has started doing activities with all of us present.
This vacation was supposed to be about me, her, and our three children coming together, not driving us apart. Our previous vacations were filled with togetherness: fishing trips, picnics, and more. I couldn't understand why she was so insistent on inviting him this time. It's not like Jason hasn't joined us on vacations before; in fact, when he was 19, we went on a vacation with him. If I recall correctly, I spent most of my time with my wife, but I'm not a night person, so I would go to bed early. On the other hand, she's a night owl, so if there was any affair back then, it would have happened when I was asleep.
I can't shake the feeling that this affair has been going on for quite some time. I suspect she took more risks this time because she hasn't seen Jason as often since he's been busy with college. Looking back, I question her frequent late-night dinners with married friends, which happen at least once or twice a week. Interestingly, these gatherings occur more frequently when Jason is on break from college.
Our closeness has always been healthy, but I've noticed her libido increasing with age while mine slows down. We still have private moments once a week, and if it were up to her, it might even be more frequent. I can't help but wonder if my reduced intimate activity played a role in all of this.