I've learned that, for me, the emotional connection of attraction is way more important than the physical part. Lots of married people have to change their views enough to understand that even if their spouse is perfectly attractive today, 40 years from now will bring plenty of changes. How many today would find someone 40 years older than them attractive? Yet I'll bet that if I'm blessed enough to be with my wife in 40 years, I'll still be very attracted to her and hopefully she will feel the same way for me - even though I'm sure I won't be that attractive to anyone else (in fact, I can think of few things less attractive than what I'll likely look like in 40 years)!
Expecting a spouse to always fit a certain mold of "attractive" is probably not good for long-term relationships. And I wonder how much of what we individually are able to accept as to physical change comes down to societal expectations and conditioning. We accept and stay in relationship with a spouse 40 years from now because we're kind of expected to, for instance, even though few would say an 80 year old is more attractive than a 20 year old, etc.
That said, obviously not everyone wants to be in a romantic, physical relationship with someone who changes in certain ways physically. I acknowledge that, even though it's not how I'd see things.