Note that the assumption they were making wasn't that you were getting married to have children, you were getting married to have sex.
You don't care about Rubella results if you're only going to have sex. You care about Rubella results only if there's going to be a fetus.
Sherif Girgis makes a pretty good case here in defense of traditional marriage:
http://www.c-spanvid.../program/310722-1