Unable is a different thing than shouldn’t. Unable is clear, shouldn’t implies that you’re defective in a way that should not be passed to next generation. This can also be factually true in case of some terrible, hereditary disease but I doubt it’s what we’re talking about.

Also, discussing fertility with other people is becoming thing of the past. It’s not polite to ask people if and when will they have children. It’s even less polite to suggest them one way or another.