That said, it's a stupid argument. If you want to two-space, fine, but be aware that most people believe and understand you're living in the typographic stone age alongside people who indent their paragraphs and use Whitesmiths.
Two-spacing is a result of monospace typewriters, which couldn't provide the "space and a half" spacing that traditional typesetting put between sentences, however these days most of the people I see doing two-spacing are in their forties or older, and probably used a typewriter before they used a computer.
Even when using a monospace font, I prefer single spacing. Why? It's easier to be consistent. If I forget to two space, or have a typo and miss a two-space, short of creating a really nasty regex I'm not going to be able to find it.