While gender segregated security lanes are not always in place, they exist. For the reason mentioned by oliver2002 . You have the right to be physically checked by someone of the same gender than you. You are not obliged to accept a check of a male security officed if you are a female passenger and vice versa. And the vast majority of female passengers want it this way. The xray scan can be operted by a male, but female security officers are still in the minority. If is quicker in this case to have gender separated lines, as two female officer working the check are quicker than a mixed line.

Sometimes you are asked if a check by someone of the opposite sex is okay, but this is rare.