Women don't get NGU. Its simply a bacterial infection in the urethra, and it can be caused by a bunch of different bacteria. NGU means non-gonoccoal urethritis, but a better name is non-specific urethritis, because many times, the cause is not determined.
A very common way to get it is through oral sex. Normal mouth bacteria can get into the urethra and cause an infection there.
Here is a good link about it - http://www.ashastd.org/learn/learn_ngu.cfm