This happened to my marriage. It became obligatory, we were doing different things on our own. I was traveling alone or with my children. In the end, we were just existing, going through our daily routines. Even though we were just going through the motions of life, the separation still hit me hard. I guess I had conditioned myself to accept that we are going to grow old together — no matter what. He ended up finding what I couldn’t give him with someone else. He’s happier, and he deserves to be happy. I like how you termed it as obligatory — because this is exactly what my marriage became in the end.