You have to consider a few things here. You're not the only one with social anxiety at that event. Your reaction is not to go in the first place (my MO as well) or to sit quietly hoping it shall pass with haste. Others talk too much. School reunions are such a rich vein for neurosis because you're guaranteed to be judged by your peers. Peers who knew you very well when you weren't a more well put together person yet. Few people behave like themselves there. So if the woman says she cheated a million times lol, her neurons may be on the fritz as well because she's more thinking about how she dunked Sharon's head in the toilet in freshman year or whatever. And that memory is haunting. And she's sitting just over there! WHY HASN'T SHE SAID ANYTHING? ... So you need to have a salt shaker handy for anything you hear.

Also, some people like attention and will say anything to get it. People like to construct a public persona around their worst character traits, the ones they're unable to change. It's like they're putting a cool leather jacket on, aviator shades too. To distract themselves from their inner monolog, which very well might be telling them what a piece of shit they are.

And cheating is common. In my social group I know of a handful of cases. Drunk and horny, sober and crushing - the motivations are on a scale. In some relationships these secrets never get revealed, in others they've made the bond stronger, others have broken up. I would say very few people brag about it but hey, we contain multitudes. Some people end up in an unhealthy game of hurt oneup(wo)manship. Relationships are hard fucking work.

It is also a different picture when you have children with your partner. The willingness to forgive infractions increases for the good of the children.

And while centuries of indoctrination of monogamy and loyalty to your spouse can make this hard to accept: some people make open relationships work. I think it's more often than not the last stepping stone to disaster but if you can make it work, vaya con dios. I have a hard time with it but I'm trying not to judge.

None of this needs to change how you feel about cheating though.