As far as I know, it is the only word where wo is pronounced as wee. What is the reason for this? Does it have to do with the origin of the word?
Well, here in the Midwest of the US, we don't pronounce it 'wee'.
In the singular form, 'woman', we use a sound similar to the word "wool". The IPA is: `wʊ̈-mən.
In the plural form, 'women', we use the pronunciation that RedDwight has in his answer, with a sound similar to the word 'it' or 'win'. The IPA is: `wɪ-mən.
So, in neither case does a long "e" sound.