And maybe he's not a cheater, maybe they have more of a DADT situation or similar until something becomes serious. However, I'd sure as hell skip having sex with him again until I'd met/heard from his partner and made sure she was OK with it.
I do know my husband and I have VASTLY different ideas about when to inform each other about what, and that has caused a number of interesting and/or stressful conversations. Always a chance his partner just doesn't need to know until something is serious (maybe he goes through partners like kleenex during a cold and she gets tired hearing about a zillion kleenex, etc).
And, maybe he's a cheater.
Happiness will never come to those who fail to appreciate what they already have.