I haven't been fortunate enough to have a long relationship myself but from what I have I observed with friends, and what people say in the dating world, this sounds pretty normal. Not the ideal, but I would say a large percentage of marriages do become this once kids come along.
Many long term relationships seem to have a 20 year lifespan..enough time to have and raise children and then to realise that in the work and joy of raising children, the couple have grown apart.
At that point, you have to make a decision....either that you are happy as you are and continue on. Or decide that you want the chance of romance again, and split up. At any time you are both free to make another choice.
But the grass is rarely greener, and the dating world is pretty horrendous. I would say better to develop your own life interests or take courses together and remain mentally interested and interesting with each other.
Some people are lucky enough retain romance and sexual connection throughout a long marriage, but I would guess that's not the majority.
My parents have been married 60 years, I would say there was a good 25 years in the middle where nothing was said of any more depth than 'We need more eggs'. They are gifted with the ability to find contentment in what, to me, would not be a happy life. But then later on life they became all cute and romantic again, albeit sleeping separately due to snoring.