Everyone deserves to marry and be with their loved one, no matter how much they earn. Stopping them from marrying wouldn't prevent them from having sex anyway. I don't think people who can't support kids should have kids, whether it be because of their financial situation or their capabilities to devote their time and effort to them, but I certainly don't think there should be a law for it. Next thing you know they'll keep making the line of "poor" higher and higher, until only the wealthy and powerful can have kids, or only special groups of people who's kids will inherit their same strength in government. Maybe someone will say that only people over 25 or 30 can be fit parents, or only people with masters degrees. Everyone deserves the right to have kids, and if this is an issue over welfare then they can simply make the welfare laws stricter for people with too many kids, rather than dictate people's reproduction rights.
So long story short, it may seem sensible, and even better for the greater good to decide who can and can't breed, until the person they say can't ever have a family is you, and I never want someone to tell me I couldn't be a good parent or pursue my happiness. My parents never depended on anyone, but they were technically poor, and we lived off the basics, but I was always happy as a kid and they taught me a lot.