You're so right. My partner has Marfan syndrome (as well as several major mental illnesses) and one of the possibilities is that if he isn't careful, he could have a shortened lifespan. But does that mean I would choose not be to be with him due to that? No! I hope he doesn't die until he's an old man, and I hope we both live to 100 together, but if he does die young, I will be so grateful for every moment I got to spend with him. Because I LOVE him. However long his life is, it's precious, and so is he. That's what "for better or worse" means. It's not a burden it's how you genuinely feel.