This pentration is not limited to penile, but can include other body parts or objects. The legal definition of rape can be tricky, but remember that even if the law is not on your side, many others are. 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.