He lied before when he approved the ads saying "Obama supports comprehensive sex education for kindergartners," and he just lied again when called on it and said "he did." He knows it's a lie, and he repeats that lie.
John McCain is a liar. Witness him, my friends, being a liar:
Here are some facts. Here are some more. This goes back to 2004 when professional asshole Alan Keyes tried this against Obama. and more, from FactCheck.org.
Did I mention John McCain was a liar?