From the research that I have done, most cultures who are ok with these sort of relationships are forcing people into it. Due to their culture, they do not have a choice. Refusing to go trough with a marriage between cousins would end up having the family denounce whomever is not agreeing. So to answer your question, I think that in most cases it is within consent or it is simply accepted in order to respect a certain culture or religious belief. The social aspects of this are quite complex.