That's not what I was taught! Two spaces after any punctuation mark that ends a sentence is, to my knowledge, still perfectly current and acceptable in the US.Using 2 spaces is oldfashioned. Nowadays only a single one should be used in English.
The usage seems to be different in the U.K. because it changed from double spaces to single spaces in the mid-20th century (see this Wiki).That's not what I was taught! Two spaces after any punctuation mark that ends a sentence is, to my knowledge, still perfectly current and acceptable in the US.
+++ EDIT #2 +++6.11 Space between sentences.
In typeset matter, one space, not two (in other words, a regular word space), follows any mark of punctuation that ends a sentence, whether a period, a colon, a question mark, an exclamation point, or closing quotation marks. […]
The traditional American practice [is to double-space after colons and periods]. […] This practice is discouraged by the University of Chicago Press.
With a computer, use only one space following periods, commas, semicolons, colons, exclamation points, question marks, and quotation marks. The space needed after these punctuation marks is proportioned automatically. With some typewriters and word processors, follow ending punctuation with two spaces when using a fixed-pitch font.