Ask him how he'd feel if you told your mom he was getting treatment for erectile dysfunction?

Why could he just tell her HE was in therapy if he needed to validate her in some way?

You can't fix stupid. So don't even try. But... you can punish it into submission.  Have a lock put on your bedroom door and let him know that he will not share your bed until he clearly recognizes that when you ask him to not disclose private marital matters that he will respect your wishes whether he agrees or understands your position or not.  Make it clear that his daugther has zero standing or place in the relationship that you and your DH share. She is a child and has no standing in your marriage.  Their relationship should not include any private matters between you and DH.

Good luck with this shit pool of genetic enmeshment.

My DH did almost exactly the same thing. I felt betrayed and he couldn't understand why. Because we agreed NOT to tell our children our personal, marital details, that's why. It's not a hard concept.

But somehow being buddies and bonding with a kid is more important than a promise to a spouse. I blasted my DH too and he learned that I expected him to keep his promises to me. Still infuriates me that I had to explain that 

My ex DH used to share our marital problems with his mini wife. All our troubles surrounded ex DH and SD, and me intruding on their marriage. It was an enmeshed clusterfluck to the max.

I would tell your dumb ass DH this is NOT acceptable and then use Togs line about erectile dysfunction. 

Why these sick men get re married is beyond me, they have all their needs met with their mini wives well except for the sex part (we hope any way  ewwww LOL) and even that is pitiful. Id rather become a nun then have sex with my ex again, so yawn worthy. Ok I'm off topic. LOL

He agreed with you not to tell anyone,  then does, then is flippant when you confront him on it.

Emotional incest includes sharing marriage things with kids...he is also putting her before you.

I would question his faithfulness to you regarding other things since he sees no problem betraying your agreements.

This is the voice of experience. Mine did this early on, and how I wish I knew then what the people on this page have taught me since. That definitely was a red flag. No matter how good he had it, he just felt the need to not walk, but RUN to her to tell her the tiniest of things. 

I do agree that enmeshment is horrible. So many good wives are made to feel subordinate, or "second best" to SD's in particular. It's as if daddy was getting all his needs met through SD and vice versa. Emotional incest is really quite perverted, and a tough knot to untangle.

Some betrayals were smaller than others, but betrayal is kind of like sin, there's no yardstick to measure it, categorize or place  the offenses heirarchically;it's all viewed the same.

Marriage to enmeshed daddies is almost like eating a mushroom you've never seen before, you don't know the danger until it's too late. 

I am so sorry you're experiencing this from the person who vowed to put you first. If I had pertinent advice, I would definitely pass it along, unfortunately, I can only com,iserate with you, and wish you the best in the future.