Maybe try to bring up in a casual conversation what the rules of their relationship are. From what I read here, I am guessing there are high odds that they weren't a totally monogamous couple anyways.
(Not to be stereotypical, but living in the gay male community for 17 years now, I've started assuming most couples are not monogamous unless they say otherwise...instead of the other way around. It's usually on the down-low, but most do something.) It doesn't hurt to start a dialogue of how and what goes on.