I think tha'ts not true, a non jew can be married to a jew, but the opposite is true: a jew can't marry a non jew. Why?
Every body know that jewish people are very closed to theirs religion, and I fully respect that's belief! so in a common religion thought, when a jew marry a non jew, there's always the possibility for the new coming ( to jewish community) to opt for jewism. But when a jew marry a non jew, the jewish community will perhaps lose an adept.
Obviousely that's not true, that depends on one's personality and beliefs freedom. But that is the traditional religious beliefs specially when it's related for girls to get married , it's strictly forbidden to a girl to marry a non jew man.
And who knews? perhaps I'm wrong. With my full respect for all religions and all beliefs.