Biologically speaking, sexual "readiness" is not confined to being 16/18/21, it's different for everyone, and so therefore and religious or social law, imposed on the masses is just a minimum age you must be for "justice" to take it's course if there is an offender on the loose, or the victim is under-age.
I believe that when you are emotionally, physically and spiritually ready for sex, then it is up to you as to how, who with and when you do it, but you must adhere to your societies rules and boundaries, for your, and your partner's sake.
Sex is in the same grey area as morality, you can't constrain impulses by law, it's different for everyone, personal morality is all we have to govern our bodily responses at the end of the day, any punishment present by society is merely a deterrent to those who would do such things, and a way to keep order amongst a large group of individual people.
So to answer your question, sex in many cultures and places is a taboo, because it is an extremity of emotional and physical responses, much like rage. To protect the society itself, these must be tempered in some way, otherwise tensions between people build, and violence happens. This can be through religious or lawful control, but is a control nonetheless.
I believe that when you are emotionally, physically and spiritually ready for sex, then it is up to you as to how, who with and when you do it, but you must adhere to your societies rules and boundaries, for your, and your partner's sake.
Sex is in the same grey area as morality, you can't constrain impulses by law, it's different for everyone, personal morality is all we have to govern our bodily responses at the end of the day, any punishment present by society is merely a deterrent to those who would do such things, and a way to keep order amongst a large group of individual people.
So to answer your question, sex in many cultures and places is a taboo, because it is an extremity of emotional and physical responses, much like rage. To protect the society itself, these must be tempered in some way, otherwise tensions between people build, and violence happens. This can be through religious or lawful control, but is a control nonetheless.