There simply is no doubt about that for the vast... vast number of people.
The worst shape i was in started about 3 years after my marriage and up until we divorced. We both got comfortably over weight (not fat).
I get it in a way. Marriage, in a way, is permission to get out of the rat race.
But it is also horribly sad if you think about it. We marry promising each other our best and yet most of us then give our partner our worst version (at least physically) with the best reserved to return ONLY if you become single again.
I have been around many, now single women, who bring up the idea of them having a revenge body that their ex gets to see but not have. I never say it but think 'he was never going to get that body anyway'. And again that goes both ways.
Sadly it does seem, for the vast, vast majority our best (physical) self is not something for our partners and is only for the person 'next'.