Either by herself or by force submission.

The penis get erected by engorge in blood. To do that these things must take place:

1-Blood displace from brain and muscle to the penis.
2-He must be relax + arousal by the woman’s beauty.
3-To relax he must be free of stress: not worry about getting caught/ get infected with STD
3a- To get 3 he must be either: stupid + career criminal
4- If #3 is not the case, He must have very high blood pressure. Otherwise, there would be insufficient of blood rush to the penis. Yet, high blood pressure men are usually erectile dysfunction= he must be young.
At least , #4 must happens in rape forbidden Western Society.

The notion of rape is ONLY about power, seem to be highly error

Conclusion: Either the women must submit herself by fear/ arousal, OR being knock out before rape take place. In either case rapists, would have to be a very rare species of men.

You thought?
If the rapist still has to struggle with the woman/ or Stressful, his blood would still reside in the arm muscle/ brain, making in a physiology impossible to get erected. Unless he is a freak, a super man.
@Arturo: I am not doing drug/ drunk. I only smoke completely sober now.
@Never trust a hippy: I can NEVER get erected without being relax/ free of fear and arouse by the woman herself.

You must be very unsual if you can w/o the above conditions satisfied.