Anna and Alex are married. Brad and Brenda are married.How you deal with female infidelity in other relationships is a shit test of sorts. By accepting it in the relationships of your social acquaintances, you are showing weakness and bowing to the female imperative. This will be taken, on an unconscious and emotional level, as being implicitly unworthy of fidelity in your own relationship.
Anna tell tells Alex that Brenda is having an affair and cheating on Brad.
Alex is somewhat friends with Brad, but Anna swears him to secrecy. Alex complies and doesn’t tell Brad. Besides, how do you tell someone that they are being cheated on. So awkward.
Anna starts to get snippy with Alex over little stuff.
Anna starts telling Alex how annoyed she is with Brenda and how she’s changing and doesn’t like it.
Anna gets annoyed with Alex over lots of different things.
Where Alex first goes wrong is not telling Anna that either she will tell Brad about Brenda's affair or he will. Being sworn to secrecy by a woman in such a situation is totally meaningless; it doesn't require even a quantum of Game to know they don't take such promises seriously. Alex then compounds his original error by playing along and permitting Brenda to remain an acquaintance; Anna's complaints about Brenda are really her subtle requests for Alex to step in, be the bad guy, and remove a bad influence from her life.
Instead, he's shown that he is too weak to resist the vagaries of female whims. Little surprise that they soon happen to wash over him and his marriage.
Learn from this. Be the bad guy. Embrace your inner Darth Vader. Turn over the rocks. Expose the insects and force them to scurry away, out of your life and out of your marriage.