How not to lose the desire for intimacy in a long-term relationship

It's not easy to come to terms with the idea that you will have intimate relations with the same person for the rest of your life, unless it's you yourself. Sometimes it can seem monotonous, but it is not impossible. The problem is that it was long believed that the key was to spend as much time together as possible to build "intimacy", which would then naturally lead to an intimate relationship. In reality, things often don't work that way.