Pregnant women (or rather, their fetuses) are most at risk, from what I have read, because the disease passes to the fetus and causes very serious problems.
Immunity for Rubella lasts about 18 years. Pregnant women like me (who were not exactly expecting to get pregnant) might find their immunity wore off 20 years ago, and the vaccine for Rubella will not be given to pregnant women, since it is a live vaccine.
http://www.nhs.uk/chq/Pages/1104.aspx?CategoryID=54&SubCategoryID=137
And where will pregnant women most likely come into contact with Rubella? Other people's children. More appropriately, other people's unvaccinated children.
So you take your little unvaccinated 6-year-old offspring to the local diner. He's just got a mild rash -- nothing for YOU to be concerned about -- and he sneezes on me. Thanks. You've just put me and my fetus at risk.
As a pregnant woman and a tax-payer I have a right to go to school, work, dinner, the grocery, etc. So don't tell me I should just stay home for 40 weeks or wear a mask and a hazmat suit whenever I step out my door, for fear of all the Jenny McCarthy mommies and their unvaccinated brats. (Although I'm sure with your attitudes towards "women's lib" that's exactly where you'd rather see me.)