I been married for a minute, that is to say almost a quarter century. I never thought about it, but it really comes down to the habits you carry into the marriage is generally what you carry through the marriage. If you (dude or chick) get into a relationship with a somebody that works out because it makes her feel good, then that's who they going to be. Conversely, if you get with a person that either is working out because they trying to catch or just look good without working out, that's what they are going to be. When that person gets older, their bodies are going to change. It would take a monumental shift to make somebody start being a gym rat and eating super healthy later in life. Certainly, things are going to change as your relationship matures. At the end of the day, the more you are into your mate, the less that shit is going to matter. My lady certainly does not look the way she did when we got together, but I'm just so into her. She can get whenever she want it. That whole beauty is in the eye of the beholder is some real shit.
BTW, holding your man responsible for maintenance of your fragile mental state is nasty work. I would run from a broad like that. There is real value in really trying to get to know people when you are dating. That means paying attention to red flags. When somebody tells you who they are, you need to believe them.