I also think with many men it's that they can't be arsed to emotionally connect, marriage isn't for them.
They distance themselves from you in all sorts of other ways too it's just that sex is the most noticeable. (More time spent at work and on hobbies, don't cuddle in the sofa and enjoy the same TV programmes, no effort for date nights or walks together, less effort put into birthday gifts and surprises etc.)
These all start breaking the bonds of connection....that go unnoticed as they are easily explained away 'he's a good dad, he provides for the family, he's a bloke he's not cuddly or chatty, we have different interests in tv and hobbies, he needs his 'wind down' time, we are all frazzled with family life'
But essentially he isn't that into you, can't be bothered to connect with you, is bored of marriage, selfish and lazy and probably not in touch with his emotions at all (this is often when some sexy work colleague or similar wanders into their life and steals their heart - their guard is down they don't even realise how unhappy they are until that point)
Why would he want to connect in the such intimate way with you when he's already checked out. Your needs are not being met at all!
When you confront him and stamp your feet with indignation he will comply for a while (for an easy life) but things still slip back.
You need to be very firm and clear, get counselling and both make changes to save your marriage and reconnect or you need to leave - both parents unhappy and not living their best life is a poor example for your DC.