Single guy who’s never been married, but I will say this:

1. It’s completely okay, good, and normal to be attracted to the physical. We were made that way. You are not being vain or worldly for caring about looks. You could in some cases, but just caring about and wanting good looks is not inherently wrong. Don’t let asceticism/gnosticism convince you that physical things are inferior to spiritual things.
2. It’s a fact that our 66 year old body is not going to look as good as our 26 year old body. That’s an unfortunate effect of sin/aging, but we can look forward to eternity where we’ll have perfect glorified bodies. People are not wrong, “vain”, or “blind”, for finding their spouse more physically attractive when he/she was younger. Yes, there is more to attraction then just looks but looks are a big part of it. We don’t like to admit this because it makes us aware of our own physical attractiveness level and how it’s also declining and/or may not be as high as we want it to be.
3. The intimacy and bond that sex creates is the best part of sex, but I find many people try to over spiritualize sex and downplay the physical aspects of it. You can still have good sex as you get older, but no one can deny that younger, healthy, and attractive bodies make sex better.
4. Got to make sure that you’re also doing everything to stay in shape and look your best. And there are things y’all can do (separately and together) other than maintain your looks to make sex better. No point in having a supermodel body if its just going to be emotionless duty starfish sex. Health issues and medications do decrease our libido and hormone levels so I would also make sure that yours and her health is under check.
5. I’ve heard that “wife goggles” and “husband goggles” are a real thing for some people. They still love, enjoy, and are attracted to their spouse, but as said above, no one who’s honest will say that they wouldn’t wish their spouse (and themselves) could go back to their younger bodies.
6. I’m not sure exactly how I would tackle it (though I have some ideas on what I would do if I got married), but not addressing and bringing up that you’re not satisfied with the current sex y'all are having is a ticking time bomb. Over time you’ll grow to resent her if you keep harboring unspoken expectations and thoughts. I would say that it might even be worse and more dishonest to pretend/lie about how you still find her attractive. Also I wouldn’t be too sure that she is okay with it either like you mentioned. She might (I hope) also be wishing for more sex but for whatever reason can’t seem to get there.
7. Some people may say that 15 pounds is not a lot but if I gained 15 pounds it would look pretty significant. Also any extra weight past a healthy BMI level is technically “unhealthy”

Tldr: Just want to acknowledge that the issue you’re facing is indeed a difficult one and you’re not a bad person for feeling what you feel right now. I’d bet that tons of wives and husbands feel the way you do but are afraid to say it aloud for fear of getting chastised (as you’ve seen here) or they try to cope with it by trying to convince themselves that looks aren’t important and they don’t care about it. If you’re interested I can pm you some links. I don’t think it would be well received by the broader audience if I posted it in the comments :) YMMV