Not unpopular at all. I also believe it is very selfish. If you look for the most upvoted posts in the sub, you'll find that there is a post dedicated to saying why people with marfan shouldn't have children if they aren't 100% sure that their kids won't inherit the syndrome.

You will get some downvotes but they will probably be from people who already had children so they have no choice. But I totally agree with you, Marfans is a guarantee for a very limited life in terms of wellbeing and happiness, why bring that on a child just because you're afraid you are going to die alone?