Flu prevention advice says "keep your hands away from mouth, eyes, nose." why are other mucous membranes not included, such as urethra, anus, vagina?

Flu protection ?: Airborne spread of the flu virus can come into contact with inanimate surfaces touched by hands, and washing one's hands after contact with "other mucus membranes" as mentioned usually will prevent spread to those areas. The real "reason" for hand washing is the spread of viral particles by infected individuals on doors, stair railings, etc..

The obvious: The "advise" is a generality based on the most common forms of community spread. Of course someone who has flu or is in intimate contact with one who is infected will get it during intimate contact. Obvious forms of spread are seldom publicized to the same level because they are obvious.

Get vaccination: Best thing for flu prevention is to get vaccinated. Wash your hands often. Have a healthy diet, exercise 30 minutes/day, drink plenty of water, have safe sex, no tobacco alcohol weed or street drugs. For nutrition: http://www.choosemyplate.gov/.