I don’t want to be shallow
100 pounds is a bit beyond “shallow”. I am 6’3”. In undergrad I ran and worked out daily, which put me at ~200 pounds. In grad school I buckled under the pressure and gained 90 pounds. The difference was utterly astounding. I went from a size 32 waist pants to a size 40; everything hurt, I was constantly pulling muscles/tweaking nerves and joints. I was tired all the time. Eventually, I shed most of it, but I’ll never forget how miserable that was. That affected me not just physically, but emotionally. I lashed out more, as it’s hard to be positive when you’re just feeling bad all the time.
I cannot imagine your wife is also > 6’ tall, so I cannot begin to imagine the physical difference 100 pounds would make on someone shorter. That’s 10 more pounds than me on someone possibly several inches shorter than me. That’s essentially a physically disabling event, and would make a person almost unrecognizable from when you first became attracted to them.
These feelings are something you have to accept. You feel them, you can’t escape them, and unless she’s taking active measures to correct the situation then it’s not going to get better, maybe even will get worse. Physically, she’s an entirely different person than you married. Emotionally, that may also start to be the case if it affects her mood as well. There is nothing wrong suddenly realizing that the way a person has changed over time no longer fits with what you want in life, but you have to accept those feelings to take the next step. IMO, if I were in her shoes, I’d want to know how you feel before you hit the breaking point and wanted to leave; she deserves a chance to decide if she wants to make a change to salvage the marriage.
There is one caveat: if this was brought on by personal choices, and is rectifiable, then that’s one thing. But if this was brought on by some medical event, and she is gaining weight because of something that a doctor has specifically stated cannot be contained or controlled, then that’s more like a cancer/other medical crisis event IMO. Then it’s a question if if you are willing to stay with her through thick and thin, even if your feelings physically have changed. I’m not talking about someone making excuses via a self diagnosed “condition”. I mean a situation where there is no doubt, doctor confirmed sort of thing.
Good luck. I’m sorry for you both. This is a tough situation for everyone and it will be a painful one to resolve.