I was watching the episode "Birthright" from television show "Criminal Minds" and had a question regarding the killer's actions when it came to women. When the BAU found Robert Wilkinson's (the killer) private journals and maps they read that he would abduct one woman and rape her in his barn every night, than after 4 weeks he would abduct another woman and save her for when the other woman was (in his and the BAU's words) "unable to have sex. When this happened he would kill the one woman and start raping the other until he had to kill her.

What I do not get is that women are biologically and physically able to have sex every night (like those ladies) if they want to. I did not know that women get to a point where they can no longer have sex. Men are the ones who usually exert the most energy during sex.