I'll quote from the LW: "My boyfriend then threw this in my face during our marriage discussion, and while venting to my friend about my boyfriend, I told her."

She blurted this out while venting. While talking about herself. While she was trying to get her friend to empathize about how crappy her boyfriend is for not wanting to get married, she actually used a non-specific weak point in her friend's marriage to prove her point. And she is surprised that her friend is pissed.

More importantly, put yourself in LW's situation. If your boyfriend/girlfriend told you something potentially incriminating about your good friend's husband/wife, wouldn't you want specifics, on behalf of your friend? Who among us would take that information (general as it may be) and just think, "Hmm. My friend's marriage has a secret. Damn my boyfriend for bringing that up, he's such a jerk for having an extra point in his anti-marriage arguments." NO. A caring person would immediately have empathy for his/her friend, and want to look out for him/her. The fact that LW didn't even care about this secret, except for in the way it would affect her, speaks volumes.