A question that hasn't been answered conclusively. You can read a paper that I wrote reviewing theories if you'd like. Assailants seek to overpower their victims. Rapists are usually strangers. Gang rape is rare.
Rape is primarily a sexual crime.
Research shows that rapists are looking for available, vulnerable targets - not women who dress in a particular way. No person asks to be hurt or degraded. It is impossible for a husband to sexually assault his wife. Regardless of marital or social relationship, if a woman does not consent to sexual activity, she is being sexually assaulted. Whatever a person does to survive the assault is the appropriate action.