@alexiooo marriage and child-making are completely unrelated things in the modern world (widespread contraception). There's adoption, there are semen banks. Also, note that according to science there are more chances of genetic diseases with childmaking after 40 yo than with blood-related relatives. So according to your comment we should also forbid marriage to all people after 40, and to people with genetic diseases in general.
If the health of the children were the real concern, it would be fine to mandate contraception, or vasectomy. It's just because the $insert-local-holy-book-title says that gay marriage or blood-related marriage are dirty. But in a modern liberal state the community can't be allowed to have any right to say who I can and I cannot love.