It's easy to say that you will stick with your partner because you love the person. BUT eventually, your priorities will change and you want to be with someone who would be there for you, who is as young as you are. Someone whom you can laugh with, have fun with and share your days without limitations and without feeling guilty of how your partner will feel. I am sure your partner assuming she/he has some deadly illness would not hinder you from your happiness. You can only do so much as her/hislover, but everything has it's limits.