If they have gained significant amount of weight you have to ask why. Maybe they are going through a depression? If they aren’t willing to try getting healthier than no I don’t think you should be shamed for not being physically attracted to them anymore. 
BUT if your significant other had a traumatic accident or disease and they CANT help it or they naturally aged and you leave them I feel like yeah you deserve the shame. LOVE goes beyond the physical appearance of someone. If you could just easily leave a person over their physical body and be ok living without them for the rest of your life then you never truly loved them to begin with. 
“Should the other person really be tied for life to look after someone” just don’t get married if that’s how you feel. “IN SICKNESS AND IN HEALTH”. Swear the human race never ceases to disgust me.