Double spaces after periods are also grammatically correct. Only in recent years did the one space become acceptable. I think one space looks like ass, and can't stand how websites force the two spaces down to one. I would never, ever submit anything written with a single space between sentences.
Double spaces after periods are only correct with monospaced fonts (like the kinds used on typewriters). Nowadays computer typesetting programs automatically place the correct amount of space after a period with only a single space. If you don't believe me take a look at any book or magazine.