>That's the function of the period,

This breaks in the case of "Mr. Jones went to the store. He enjoys buying things." The spacing after 'Mr.' should be less than after 'store.'. If two spaces are prohibited, you have the following options, neither of them particularly satisfactory:

-The user must input some form of special Unicode space character after 'Mr.'

-The display software must include a list of all words that, when followed by a period, should not include extra space (e.g., "Mr, Mrs, Messrs, etc, e.g, i.e" and so forth).