In the west, one can shake hand with a woman, dance with her and even kiss her on the cheek, and none of that is considered to be sexual. But one must not touch her in her crotch. That is private part.
A Muslim woman’s entire body is private part. So if you shake hand with her it is the same as touching her crotch. If she displays her hair, it is as exposing her pubic hair. Everything in her is sexual and private part.
In Islam women are the namoos (?????) of their male relatives. Namoos can be roughly translated as honor, but with a sexual connotation. The namoos of a Muslim hinges on protecting his female kin from prying eyes. If a woman displays her beauty or talks to a stranger man, the namoos of all her male relatives is injured. The only way to redeem an injured namoos is to wash the stain with blood.
http://formermuslimsunited.americancommunityexchange.org/2010/02/10/honor-killing-is-about-islam/