Why do some women use intimacy as a bargaining tool?

During the short time I’ve spent with my girlfriend, it seems obvious that romance and passion are low on the list of reasons why she gets intimate with me. It almost seems like she does it to get it done with or because everyone else is doing it.
Women like to use intimacy as a bargaining chip. Net photo.
Women like to use intimacy as a bargaining chip. Net photo.
Times Reporter