She probably has attachment issues. A lot of women can't get hot and heavy for a guy once their needs for security are met. I know this is really frustrating for you to go through, but keep in mind she is probably annoyed at this as well. She probably wishes she could desire her husband more than she does. If she is like other women she is prideful and will never admit to anything being wrong (like you said), so most likely she is internally shaming herself for being defective and broken.
Most likely this has something to do with her childhood or her father, or her feelings of low self-worth. She also may have been sexually assaulted in the past, and has alienated herself from her sexuality. She was willing to "grin and bear" through sex to get children, but now that her goal of children has been met, what is the point of her having to "put up" with sex anymore. Kind of like how you stop showing up to work once you have enough saved to retire.
She may be afraid of intimacy, she may be bored, she may no longer find you attractive due to something you've done in the past but doesn't want to bring it up because it will make her look petty and immature.
Take everything I'm saying with a grain of salt, because you're probably older than I am, but I've been through a few relationships and apparently my specialty is traumatized girls, so I know a bit about them and how they operate. It seems like the recurring theme in my problems with women is that they don't actually want to say what is going on because of how it will be perceived.
I went through a dead-bedroom at a very young age and the only thing that "fixed" it, was me totally checking out of the relationship. I didn't cheat or try to dread game her, but for some reason she became very interested in me again, and actually started cooking for me every night. The sad part though was that this in a way almost made me more angry and now no longer interested in her, because it became a game of "you only want me when I don't want you".
So with that being said, there could be a number of reasons she is no longer interested in sex. You may or may not be able to get to the bottom of this, a lot of it has to do with whether she wants to change for you and for the marriage. She may also have post-partum depression. What is her support network like. How is her relationship with her mom, your mom, and the other females of the family? A lot of times having children can be traumatic, because it brings up old childhood memories of our own.
Some books I would recommend reading in the meantime are "Come as You Are" by Emily Nagoski, "The Missing Dimension in Sex" by Herbert Armstrong and "Women's Infidelity" by Michelle Langley (Very hard to find in print, but the .pdf is easily found online, and well worth it)
I guess my final piece of advice is try not to take this situation so seriously. I know from first hand experience how frustrating it can be, but it is easy to get tunnel vision and make this problem the primary focus of your life. I would just back away from sex for a while and see how she reacts. Try to keep a sense of humor and perspective in this situation, and have some compassion and empathy for her. She probably feels broken and confused but is also afraid of owning up to her emotions. Also pray. Pray, pray pray!