According to a new sex survey, 40% of couples have sex three or four times a week . . . one in four have it a few times a month . . . 7.5% have it every day . . . and 4.5% NEVER have it.  More importantly, 43% of men and 33% of women say they're keeping a MAJOR secret from their partner.  That's two in five men, and a third of women